Rattle Noise help

[FLYNAVY]

I've been noticing this on occasion for a while now (description below). Car is an '08 DCT e92 M3....

- Random rattle seeming to emanate from under the car, hard to tell how far forward it is, but seemingly the front half
- Doesn't seem to be related to engine temp
- Seems to only occur in 2nd/3rd gear
- First rattle will be somewhere in the 1000-1500 RPM range, sounding metallic and somewhat like a driveshaft with some play in the linkage/trans output shaft
- Second rattle is a little more sustained (1-2 sec), and will occur around 2500 RPM plus or minus a couple.....it will completely cease prior to 3000 RPM and will not occur again
- Neither seem to be in phase with the engine, and it is not a sound which increases in intensity or frequency as the engine accelerates
- Not audible with windows up, mildly audible with left window only open, and most pronounced with right window open.
- Of particular note, the car will not make any of these noises if transmission is in neutral and the motor is simply rev'd. Only occurs under actual driving conditions, with the engine/drivetrain under load.
- Does not seem to be particularly isolated to hard acceleration scenarios.....I generally drive like a grandma and it still happens during normal acceleration as well
- Does not always occur

That's the best description I have at the moment. I'm familiar with the rod bearing problems folks have had (as well as the possible issues with '08 MY in particular), and while I haven't seen one of these motors do it in person, after watching videos on the boards and youtube, the sound I'm hearing is different in tone, as well as frequency (does not accelerate with the motor, does not occur across a wide portion of the rpm band). Also, since it doesn't occur in my garage in neutral with the windows down, in my mind, this would also seem to rule that out. What I'm thinking is that it is just a pure random rattle, or potentially a transmission, driveshaft, or differential problem (though seems to come from front of car, which could rule out diff). Thoughts? Anyone else have or hear of a problem that generally matches this description? Have been searching around, and am not finding much that accurately describes what I'm hearing. Thanks in advance!

[pbonsalb]

You could check heatshields and whether the exhaust is close to touching anything. I would definitely check your engine mounts, which may be collapsing due to age and causing the drivetrain to come closer to touching things or simply transmitting more NVH.

[FLYNAVY]

You could check heatshields and whether the exhaust is close to touching anything. I would definitely check your engine mounts, which may be collapsing due to age and causing the drivetrain to come closer to touching things or simply transmitting more NVH.

Thanks for the response! Yeah, it has been kind of hard to troubleshoot since I can't recreate it without driving down the road. It definitely has a bit of an "exhaust" sound to it, so that sounds plausible.

[aus]

Is it a slapping sound or a rattle?
Have your rod bearings been changed out yet?
Since you have a 2008, get an oil analysis at Blackstone.
.

[FLYNAVY]

It is a really inconsistent rattle, as outlined above. Very different sound than the noise of a failed rod bearing, at least as far as I can tell. I got a Blackstone reading maybe a year or so ago, after I had already started noticing the noise, and there was nothing alarming from their analysis.