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.