Strange Rattle?

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Car is a 32GTR. Just wondering if anyone can give me some idea on what this rattle might be. I've already searched, but nothing seems to describe my symptom accurately.

The rattle sounds is a bit like when you leave your mobile phone on a hard wooden table and it vibrates, but with a more metallic sound.

- Does not seems to happen when I free rev the motor (i.e. gear in neutral) or at idle.

- Happens on and off, but occurs 80% of the time.

- Happens between 1000rpm - 2000rpm, but usually at around 1700rpm.

- Only happens for like 1 second, 2 seconds max.

- Seems to happen when motor is under load and in most cases, when the car is just going from standing still.

So to put it into a typical scenario:

I've stopped at the traffic light, car idling fine at around 1000rpm, light turns green and I take off nice and easy, rev hits about 1700rpm and the rattle comes on for a brief second, then goes away when the car hits 2000rpm.

The thing is, if I take off faster by reving the engine harder, then it either does not rattle or maybe it does, but happens too fast and I don't hear it. Based on info that I found on this forums and the others, it seems that this could be an indication of either

- Engine pinging

- Exhaust manifold shield rattle

- Early signs of spun big end bearing

- Turbo shaft play

What do you guys think? Any similar experiences or anyone knows what it might be?