I'm in the process of doing this to my gst. From what I've read on this site as well as other forums, new sound deadening material on the floor pan, trunk, doors should do the trick. Gonna have to start ripping your interior apart to do it.

They are loud, you should give it to me.

motomattx said:

They are loud, you should give it to me.

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I second this motion, but since I'm closer, you should donate it to my cause!

But in all seriousness, coat the wheel wells with the rubberized undercoating they sell in a spray can at most auto stores.
Use something like FatMat or Dynomat on your floor pan, under the rear seat, and especially in the trunk area under the spare and anywhere you have bare metal.
If you want to be cheaper, go with the peel and stick style like FlexSeal for windows.

I have this stuff in several vehicles, but have yet to do my 99 GST.
I haven't had an issue with the fresh Asphalt smell yet in any of the vehicles I've used it in, however, I do enjoy that smell so if it ever does arise, I won't complain.

I may actually splurge and get some closed cell foam matting just for more sound deadening as opposed to the softening of road noise. The car is SO LOUD! But that is really a big part of the charm of these beasties.

I actually purchased kilmat from amazon and installed it in the doors and trunk and the doors make a hugeeeeee difference in sound for sure especially when closing and opening them. The trunk still sounds a bit hollow so I’m going to use more material and fix the cover as well.

Dang! No second/third DSM for us haha!
glad the addition of sound dampening material helped!
The kilmat is the urethane backed aluminum stuff, right?
What thickness did you use?

I wrapped my entire interior in aerospace sound deadening. See my build thread.

The rear wheel wells have stock rubber mats you will want to take a heat gun to those and remove them.

I wrapped the roof, firewall, floor pan, side walls. Everything except the doors. Doors actually have styrofoam in them for sound deadening that's efficient. Better tires also help with cabin noise.

I have solid motor mounts so my engine is loud, like before sound deadening my ears would pop loud. Now it's definitely tolerable and music drowns out the rest

Try replacing weather seals too, if you can find any, or modify seals from other cars.

Dreams Realized said:

I wrapped my entire interior in aerospace sound deadening. See my build thread.

The rear wheel wells have stock rubber mats you will want to take a heat gun to those and remove them.

I wrapped the roof, firewall, floor pan, side walls. Everything except the doors. Doors actually have styrofoam in them for sound deadening that's efficient. Better tires also help with cabin noise.

I have solid motor mounts so my engine is loud, like before sound deadening my ears would pop loud. Now it's definitely tolerable and music drowns out the rest

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about how much material did you need? I'm planning on changing my carpet in my 97 GS and wanted to add sound deadening but don't know how much i should buy

I bought 100sq ft and had probably about 30 sq ft left over.