I have quite a bit of engine and drivetrain noise inside my Gulfstream Supernova Super C motorhome. This is a truck chassis and so the engine and drivetrain noise are in front or under me. Not terrible noise, but the stock rubber mat can be improved upon
I decided to use some hot rod ingenuity and do the same as what I use on my hot rods. I covered the floor with a sound dampening material. Like Dynamat, but I used a cheaper generic version. Basically this is a high solids rubber with an adhesive and foil covering. The people that do the high end stereo stuff use this material for sound dampening and improving acoustics. It also works great to minimize noise transferred through the floor and other areas to the inside of the vehicle. That was my intent. You can find this stuff for best price on ebay or similar. Dynamat is a name brand and there are other generic versions like Fat Mat or other names. Just be sure to get the aluminum backed material. Not the rubber roof coating stuff, the foil helps it conform to the contours. You will need about 50 sq ft or maybe more. Cost will be around $100-200 depending on your source. The theory is the extra weight will dampen the vibrations and minimize the noise and the floor transferring the noise (and heat, it does have some insulation value). You can double the thickness, don't worry about being cheap, use it all up! More thickness is only better.
Once into the job, cover all that you can reach.
I have used this material in my hot rods and it works good to minimize engine noise and it also has decent insulating ability. It is not intended as insulation, but it does help a lot with heat transferred through the floor or firewall. I just had to replace my A/C compressor because of catastrophic internal failure of the compressor and boy was it a hot ride with the heat transferred through the firewall and floor without A/C on a recent 100 degree day returning home. I fixed the A/C, but finally got motivated to do the extra floor noise dampening material.
So here is the process. First clean out the interior to get to bare floor. This is basic unbolting and removing stuff.:
Make sure your floor is clean. Wipe with a wet rage or even solvent if something strange is on the surface. You want clean surface to bond to. Get your shop vac and clean it out, and wipe it down. Next, cut and fit the dynamat type stuff and put it everywhere you can. It is a pain to cut and fit, your fingers will turn grey from pressing the aluminum backing. Use an cheap expendable razor knife to cut the material, it will get all cruddy from the rubber. Cut clearance for any threaded holes or bolts. Press and roll it into the contours with a wallpaper roller or similar. When done it looks something like this:
I also took the foam and plastic cover off the engine, and removed the foam. I covered this as well with the dynamat material:
Use some contact cement to glue the foam back onto the cover. This helps the noise that comes through the cover and is probably a source for much of the noise in the cab.
Then just put it all back together, just reverse of the disassembly process. It took me about 5 hours total start to finish. It does make you twist and turn to fit it up under the firewall area and floor. So be sure your back is good shape!
Drink a cold beer or two to celebrate the job being done
I did not drive it on the road, as it was end of Sunday and need to get ready for the week. But it does seem to be quieter inside the cab from just starting the engine up and letting it run. I look forward to a real world test on the road. This is not a hard or expensive job, but it does require labor and the Dynamat type material, so figure $100-200 and a good afternoon worth of work.
This can be applied to any model MH. It works great for noise control and I recommend others to do the same