really annoying rattling, like heat shield or brake dust shield from under the car...
 

can only hear it under 20mph, over less than perfect pavement. as the car goes faster either it stops rattling or the road noise is drowning it out. can't hear it with the windows up, only down. having trouble tracking it down as it's only when moving.

any ideas? 2003 S6 fyi

Mine rattles all the time : ( I can't wait for the replacement cats to arrive.

My AC compressor pulley rattled until I fixed it.

Could be a CV joint

Thanks, I'll check those. I hope it's not CV joints, but I don't think it is...
 

it sounds like buzzing metal, something not tight. like the heat shields on the cats sounds like a culprit. i hope not.

Splash shield for rear brake disc?
 

It is not hard to bend these a little when removing and installing the wheels and they only need to move a tiny bit before they start rattling against the disc or the caliper. You should be able to bend it back with your bare hands.