Rattle at 1800 RPM and Idle/Park sometimes

[LittleViktor]

My 2004 BMW 325xi makes a strange rattle noise, which seems to come from under the car. The noise starts about 10 minutes after driving, so once the car is warmed up. Mostly, the rattle can be heard around 1800 rpm, and only happens under load. If you try it in neutral, it doesn't happen. The sound is not coming from the engine, which sounds perfectly fine, and there is no loss of power.

Also, recently after extended driving, I have heard the noise when in drive waiting at a light, and also once after I parked. This let me jump out to finally find source of noise: it seemed to be coming from inside my exhaust pipes. I pushed my foot against the pipes and muffler to see if it as vibrating, but that was not the source.

Any idea what this could be? Is it something in muffler? Or could it be a loose catalytic converter heat shield? Or a busted catalytic converter? I hear the noise coming from the exhaust, but it could be originating anywhere along the line. Once again, it only happens once things warm up.

Thanks, this would be a lifesaver.

 

[Kirby]

I'm thinking about a heat shield that is either loose, or expands when it warms up and is just touching something. A clue here is the fact that it ocurs mostly when the car is in gear. The motor and transmission position shift slightly in gear due to torque, and the the downpipe and cat are bolted to the motor. It's going to require a close look underneath to find the problem.

 

[ts03m3]

Get under the car when cold and start tapping all the exhause shields, you'll probably find one thats loose and causes the ratttle under different conditions.

 

[LittleViktor]

Thanks guys. I live in the city, so I have to take it to a mechanic, but I'll let him know where to start. I just hope he can reproduce the sound in the shop, so then he can find it.