Knocking Noise

[95skyline]

My car has developed a knocking noise under the passenger seat. It is a hollow knocking sound that only happens when you go over a bump, almost like a Knock, knock, knock. It can also be felt through your hand if you reach behind the passenger seat while driving over a bump. The noise started around two months ago and I assumed it was the actual passenger seat being a little lose internally (as the four seat mounting bolts are tight), but it isn't.

The sound began about a month after installing a one piece dump/front pipe plus high flow cat. I have checked and rechecked these bolts and all seem to be tight.

I have accepted the sound and have moved on until today when driving home the knock started getting more noticeable while traveling over bumps. Once i stop at the lights the sound become louder and started to vibrate at idle under the passenger seat. One or two revs later the sound went away, but i am definite now the sound isn't coming from the passenger seat but underneath it.

Has any one else had a similar experience as I'm now unsure what it is or what to do. Any help would be greatly appreciated.

[Sambo33gtst]

As you well know the Dump Pipe runs directly under where your passenger seat is so its very likely it will be related to that.

Stupid Question. Have you checked the Exhaust Gaskets on the dump?

[95skyline]

The dump/front pipe is an JJR item and the cat is a Xforce, both from Just Jap.

I have checked all relating bolts to these at least three times as i originally fitted the system myself. Each time the bolts have been tight. I have just got home and am waiting for the exhaust system to cool down so I can recheck them again, but i guessing they are still tight. The new vibration noise on idle is what i am really now concerned about.

[DECIM8]

Check the exhuast hangers. Whole exhaust could be hanging from onel.

[Duncan]

sure sounds like the exhaust, dump, heatshield something is touching.

[_Scotty_]

I had an occasional knock from that area that happened going over bumps, turned out to be the rear passenger seat bolt hitting the exhaust, which was slightly too close. When the bolt was removed you could see a mark on the exhaust where it had been touching.

[217/r33]

i've installed a jjr dump pipe on two r33's few months ago with no problems. double check the bolts and inspect your hole exhaust. if you can, put your car on car stands and get underneath it. shake it around to see how much play it has, you may need to replace the rubbers that hold the exhaust on.

[(B1) MR_fanny]

as stated its probably your exhaust knocking on something. Ive had it happen to me. jiggle your exhaust with a bit of force up and down left to right to find where its knocking.

[95skyline]

The exhaust has cooled down and I have checked all the bolts from turbo to cat and each and everyone was tight. I have shook the exhaust and all seems to be very tight with no abnormal movement / play what so ever.

I have basically check all nuts and bolts under the front passenger floor panel, anything i could get a spanner / socket onto including the mounts that hold the gear box together. All were tight.

The new front exhaust actually has more clearance to the chassis than the stock original one.

I think you guys are right, the exhaust must be knocking on something, just i can't locate what it is.

I haven't had a chance to jack up the rear section and have a good look underneath it as i have concentrated on the front section as the noise is definitely under the passenger seat. I might have to now try the rear as well. Even though it isn't loose as i have given it a good shake as well.

Thanks heaps everyone for the fast replies . They have been much appreciated.

[super_s13]

Might be silly but when i got a rb20 put in my silvia. The mechanic did not do up the bolts on the little half circle bracket (possibly the centre bearing not sure its exact name) and sometimes it would hit the bottom of the car on bumps and what not until it fell off at speed and made some real bad slapping noises. Check that. Might help. Other than that do turbo exhausts have flex pipes could be something to do with this if it is solid mounted from engine back.

[Sambo33gtst]

Did you have the heatshield removed when the dump was installed? If so you tend to notice alot more reverb noise in the exhaust if there is a knock further down the system (ie: past the cat) and that's why you'd notice the sound under your passendger seat.

I remember when I did my dump the clearance from the dump the chassis wasn't great so thats definately something to look at.

Im not sure if you understood my question well enough, the bolts may well be tight, but when you did the dump did you replace the existing gaskets with new ones? When the exhaust gets hot, there is more play in the gaskets, if they are old and the seal was broken with a new pipe there will be chatter in the adjoining pipes. Worth a look.

[95skyline]

The heat shield that covers the turbo was put back in place once the dump/front pipe was fitted. All new gaskets were fitted on both sides of the cat plus gasket cement or whatever it is called. I reused the original factory dump pipe gaskets as it was in very good condition and used high temp exhaust gasket cement on it as well. There doesn't seem to be any black marks left there from exhaust leaks on either item.

I did leave off one of the bolts that covers the turbo heat shield (the bottom one closes to the front of the car) as it was a real pain to get back on. Might have to go back and put it on. Thanks for the enquiry SamboR33gtst. You make a good point about when the exhaust gets hot, the gasket will expand and create more play. The sound definitely is louder once I have gone for a long drive.

I have checked the half moon bracket many times and it always seems to be tight. Not sure what a flex pipe is super s13.

The dump/front pipe is one piece all the way from the turbo to the cat.

I have now had a chance to check the rear section of the exhaust. The cat back is a Trust PE2 fitted new originally when i got the car four years ago. I have check all mounting brackets that hold it in place and all seem to in good working order and condition with no abnormal play.

By the way the suspension is still stock only the wheels have now been changed to 18's.

[wilorichie]

Just a little FYI, I had a rattle in my car... Everyone thought it was from the drivers air vent but it turns out it was a piece of plastic on the outside which echoed through to the vents (6/6 people all agreed it was coming from the right vent).

What I'm saying is, the sound came from something completely different to what everyone thought... So check around!

[95skyline]

I finally found out what the noise was that was coming from under the passenger seat.

It was the passenger seat rear mounting bolt, the one closest to the centre console. The bolt was touching the rear cat back section of the exhaust.

The seat bolt was only a couple of millimeters from the exhaust and once the car warmed up it started to rub and and grind against the seat bolt.

This sound only started once the 3in JJR dump/front pipe was fitted in combination with a x-force cat (all from just jap).

The new front section of the exhaust must have only been a couple of millimeters out at the front that made all the difference to the rear cat back section where the rubbing occurred.

The moral of the story that I learnt was from now on I only buy Japanese or good quality Australian made after market products.

As the "100% bolt on products" from china and the like have a great price tag but can at times give you more trouble than they are worth.

You get what you pay for!

[_Scotty_]

I finally found out what the noise was that was coming from under the passenger seat.

It was the passenger seat rear mounting bolt, the one closest to the centre console. The bolt was touching the rear cat back section of the exhaust.

The seat bolt was only a couple of millimeters from the exhaust and once the car warmed up it started to rub and and grind against the seat bolt.

This sound only started once the 3in JJR dump/front pipe was fitted in combination with a x-force cat (all from just jap).

The new front section of the exhaust must have only been a couple of millimeters out at the front that made all the difference to the rear cat back section where the rubbing occurred.

The moral of the story that I learnt was from now on I only buy Japanese or good quality Australian made after market products.

As the "100% bolt on products" from china and the like have a great price tag but can at times give you more trouble than they are worth.

You get what you pay for!

Just as I suggested above. If anyone else is having this problem it's easy to check, if you remove the bolt you can look through the hole and see if there's marks on the exhaust. It's not just an issue with gear from China, my dump\front pipe is CES.

[jeffworld2]

its more to do with getting bits from everywhere than getting "cheap". your catback was fine with the standard front pipe, and the jjr front is fine with the standard catback.

glad you found the problem cut it shroter and you are done

[OVADRV]

oh what a relief i thought it was my cat.

[95skyline]

its more to do with getting bits from everywhere than getting "cheap". your catback was fine with the standard front pipe, and the jjr front is fine with the standard catback.

glad you found the problem cut it shroter and you are done

You make a fair point of mixing and matching products and expecting them to all work in complete harmony, regardless of where they are manufactured.

Thanks to everyone that has offered advise. I'm just so glad everything is now back to normal.