When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
On the TECH forum they determined that the best noise reduction material they had found was divorce papers. I don't recommend that but I have used Dynamat, available through Crutchfield & other stereo dealers, with good success. It's a little spendy though.
I've used Dynamats in the past (C4). Funny you posted, bought some at lunch today with a buddy who has a reasonace problem in his C5. Just doing the back deck on C4 cut noise by 1/4, would have been better had I done wheel well covers
Sheets are 4' X 24", $35 each, pricey but easy to install. Back deck area should take 2 to 3 hours.
I used dynamat in my jeep wrangler (about as sound-proof-less as you can get) and it worked well - but it is expensive. The plus is it is very easy to use (sheets that are adhesive) and easy to trim etc.
After installing Borla Quad tips, I loved the sound but started getting some headaches. Someone suggested a $99 3 inch foam partition from Ecklers. It has a C5 and Z06 logo too! Works great! No headaches, great Borla sound!
I have used a material called Acumat in a Mustang and it works awesome. The before and after difference was amazing. I have also used Dynamat before, but it requires a lot of heating and dosen't stick very well. Acumat is peal and stick and they also offer three differen't types.