Sorry .... I know there have been many threads on this but the search feature won't bring anything up...so here we go.
My 98 coupe is noisy inside and I would like to add insulation or some type of sound deadening material. Any suggestions on what to use and where to purchase ...

I have said it before and I will say it again.......this is the best internet site...bar none...especially for Vette
Mark

 

I used Mike Mercury's instructions for the Frost King available at Lowes, it worked GREAT. For the $ this is one mod that can't be beat.

 

I also used the Frost King.

 

I don't want to beat a dead horse so I will paraphrase the usual mods (specifically the ones I have done). These were all developed by other members and there are how-tos with pictures somewhere on this forum. What you are trying to eliminate is road noise: sound created by the tires rolling on the road. Exhaust and wind noise are miniscule by comparison.
The most popular seems to be using Frost King duct insulation, available at Home Depot, Lowes, etc. This is a metal foil with about an 1/8" of foam insulation attached to it and is sticky on one side. basically you remove the carpet from the vehicle, either just the rear or all of it and lay down a layer of the Frost King. It is easy to cut and pretty cheap. Personally, I put down a layer of Dynamax Extreme before I put down the Frost King. Dynamax looks like a thick metal foil with a 18" layer of tar like (actually a polymer) black material. Dynamax excels at dampening vibration as well as sound. I felt a layer of both was beneficial. In my case I covered the entire interior rear with both along with Frost King all the way to the footwells with Dynamax on the center console and behind the seats.
The second major area involves taking the rear tires off as well as the exterior rear fender liner. This exposes an area where you can put a layer of closed cell insulation (looks like silver colored bubble wrap) between the inner and outer fender liner. You have to use glue or tape to mount this insulation: I recommend aluminum duct tape. Anything you use in this are will have to be able to resist water as it will be exposed to the elements. While the liners are out, you can place fiberglass batting in an area in front of the rear wheels. This area acts as a sound tunnel from the rear tires to the back of your head so the batting functions well as sound insulation from the tires.
The last thing you can do to reduce the noise level in your vehicle is switch to non-runflat tires. They are noisier that regular tires. Whether this is viable or not is up to you and your situation.
I will vouch for the Frost King(+Dynamax) and fender liner mods. They significantly reduced noise in my Z06. Even after the GHL Bullets, at cruising speed it is as quiet as my wife's Maxima.
Good Luck

 

Go to the Tech Tips and you'll find a couple write ups. I'm somewhat in the same position and am getting a COMPLETE Dynamat Extreme covering finished up in the next couple days. We'll see how much improvement. After fooling around with the material I'm a bit doubtful just how much effect I'll realize but there's no better way to find out but to get it finished.

 

As stated above...check out Mike Mercury's site...great photos showing the basic approach. Once the rear cargo carpet is removed, you are down to bare composite material. Once the seats are removed, and the center console (if desired), you are down to bare composite.
One then lines the floor, etc. with material. A sound dampener like Dynamat extreme, or Brown Bread, or B-quiet's Extreme will reduce panel vibration and sound transmission. Adding a second layer as a sound barrier really makes a huge improvement. I used B-quiet's Vcomp with great results. Some have used Frost King as the second layer.
Some have used Frost King alone...which functions as both a dampener and sound layer (though not as good as two separate material layers). The Frost King seems to be the least expensive approach.

Here's a link to my B-quiet Vcomp install over the dynamat extreme.

http://forums.corvetteforum.com/showthread.php?t=989605

It seems like a big project...but actually was quite easy to do, and does make a huge difference and worth the effort.