Over the past 10k miles, when the engine is at idle, I have heard some sort of rattling in the engine bay. The RPMs are smooth as silk and there's no misfiring. When the car is in gear (and stopped), it's louder, and if it's in N (and stopped), it goes away a little. It's not noticeably louder when cold, if maybe a very minute amount. I had it checked out by a MB mechanic a few months back, and he said it definitly was not the timing chain. I don't know what to think? I have checked everything and everything seems tight in the engine bay.
Oh and another thing, when I open the oil filler cap, it's a little louder (seems as though noise coming from in there)... which makes me think it would be the timing chain? But I've always had frequent oil changes, and I'm now @ 100k miles. I know M104 motors aren't likely to have chain failure... but I'm sure it's possible. And I do drive my car hard....
I am still figuring out a way to try and record the sound of the engine @ idle and post it. I suppose I could try using my laptop and then upload it to the internet somewhere??

Thanks for ANY insight.

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)