Just installed the Stillen exhaust at Mondo Car Repair in Twinsburg, and I have the same problem with rattling against the crossmember. It must have sagged a little since when the car was up we had at least 1/2 inch of clearance. Anyone have any suggestions? I read in this link someone used tire rubber. Isn't going to melt?
BTW, two of the factory nuts - one on each side, on the front of the factory cat were installed incorrectly by the factory. The nuts weren't on straight and were a bitch to loosen. My mechanic broke several bits trying to get them loose. Limited clearing around the front top of the cat did not help either, but he got it done eventually.
**UPDATE**
Rattle drove me nuts, so I bought some hose clamps, similar to the ones that tighten your stock intake tubes but a little larger, and lifted the back of my exhaust (near the flange right before the muffler on the left-hand side and also around the exhaust tube on the right-hand side) just a tad. I used the cross support bar as a lifting point. No more rattle under heavy acceleration or when hitting bumps. I could not be happier!
I plan on adding a pair around the middle support rubbers as well, just in case. Anyone know if lifting the exhaust on the rear sway bar will cause any issues? The full weight won't be hanging on the sway bar.
Too bad that when you spent $2,000 on an HFC, exhaust and installation, you have to resort to $1 hose clamps to get it installed properly. Followed all directions and had room to spare after install. Oh well, I'm happy now.