Where exactly? Do you mind posting photos?
I should have taken pictures while I was under there, but I started doing that aspect of the job as an afterthought. There weren't many locations I worked on.
On the right (passenger in the U.S.) side of the fuel tank, there were a couple of lines that could rattle against the tank. Probably return or evap system related. I put some foam between the lines and the tank.
Most of the aero under trays from golfdave's writeup mount quite solidly. However, the
rear exhaust cover is a little flimsy. I put some foam on a corner and one of the upper surfaces to reduce flexing / vibration.
I have a Strafe Design rear diffuser on my GTI. When I bumped my exhaust system with my hand, there was a little vibration between the diffuser and the corners of my rear valence. I addressed that as well.
I ensured that all of my exhaust hangers were mounted correctly and that the rubber was in good shape. The rear brake lines are hung on wire clips. I made sure the lines were clipped all the way in, that the clips were bent closed to hold the lines and that the clips were bent up, away from the road and any debris / out of the airstream.
There were a couple other small spots, but every car is different. Jack your car up, crawl under and start knocking on things. Any rattles / interference quickly become apparent.
