You may have seen one or a few other threads for getting rid of, or for lowering cabin noise.
A very detailed DIY write-up is:
However, in that thread and in other threads, it has not yet been shown how to reduce the noise entering through the firewall. Thats what this is all about.
If you quickly and easily want to see how transparent the firewall actually is for noise, you could lightly tap the firewall where it sounds like a tin drum. Then have someone else tap it ever so lightly while you sit inside. You will see and hear!
Reducing noise entering the cabin through the firewall
1) Flat screwdriver, medium sized.
2) Pliers, pointy tip, medium sized
3) Hot air blower or if lackng, a hair dryer
4) ratchet, size 10mm
15 to 20 self-adhesive bitumen or asphalt tiles, about 2-3mm thick, and 10 by 20 cm in size, or whatever other size you prefer.
Getting it done
Firstly, raise the hood. Then using a socket wrench (ratchet) size 10, remove the airfilter box and the resonator. The rubber air hose and the plug for the airflow sensor has to come off with it. Now there is a lot more room.
Next, the wireing harness has to be removed where it covers the firewall. It crosses over the firewall, under the tray just below the windshield wipers. There are three different sorts of clips holding the harness. the picture below shows the first sort of clip. After carefully cutting the cable tie, the entire clip can be unscrewed from the thread holding it. The clip will be re-used, and a new cable-tie will go through it and around the harness. see picture for details.
On the bottom of the tray, just above the firewall, there are different clips, which can easily be uncliped. The screwdriver is touching the "tongue" just before pushing it up to loosen the clip. See the following picture.
The third sort of clip holds a vacuum hose near the charcoal canister, it can be unclipped too, and then the clip unscrewed from its thread on which it was pushed on to. We need all clips again later on!
After we have made room and have removed the harness clips from out of the way, we can continue with the covering of the underside of the tray. The are a few plastic nuts to be unscrewed, easily done by hand.
It will now look something like this, the yellow underside of the brittle tray covering is seen below:
Now the firewall covering has to come off. Beware, it is brittle! The part on the right hand side in the picture will not come off without an enormous time-consuming effort! That is why I "only" took it off as far as possible, leaving the piece between charcoal canister and driver side fender on. I had to cut it with sharp knife, see where in the picture. Thats better than crumbling the brittle covering into pieces. The following picture shows the result.
The worst "noise zones" of the firewall are marked red in the following picture:
After tiling the firewall with one layer of bitumen tiles, I put a second layer on where it was particularly loud when being tapped on. These are the red zones in the picture above. With a hot air gun you can soften the tile before sticking it on. Once it is in place, push it hard using a rag, so the warm and soft tile permanently takes on the shape and contour of the humps and grooves of the firewall. After doing the firewall, do not forget to do the underside of the tray between firewall and wipers.
If you want to do an even better job, you could also do the inside of the tray, especially important is the side where it faces the inside of the car.
The harness not back in place yet, the firewall looks like this with tiles on:
I won't go into putting everything back where it belongs, because it seems self-explanitory and logical. The OEM firewall covering must go back properly, because it insulates the tiles from any heat. I used some water-soluble regular carpet glue for securing it in place, so that there are no cavities between tiles and covering.
The result is well worth the effort if you have already silenced the other parts. Doing the floor on both sides, including as far up as you can get under the dash is most important, just as the roof. Remember, any noise deadening must be thorough and systematic to achieve the desired effect!
If you haven't done it yet, the following thread shows you how.
Although the cabin noise has been lowered more than half, there still remains something in the rear that resonates the engine noise in fifth gear at about 90km/h or around 2300rpm. Also there are remaining wind noises, particularly annoying from the area where the roof meets the top of the windshield. It gets better if the slit between windshield and roof is taped.
Of course it is good to know when to stop with all that noise deadening, when the effort stops being worthwhile. Looking back after all the work outlined in this and the other thread has been done, I must say it was worth the effort. We should not forget these cars were made to be light weight and cheap, they were not made with any more than a very basic comfort level in mind.