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Any ideas for ways to help cut down on drive train noise that will not promote rust by trapping moisture?

I've added gasket seals to the dog house. Horse padding in the rear rusted the bed within a season. I'm afraid carpet will do the same.

Something like Dynamat if installed properly should not trap moisture.

I think most things put directly on top of rear bed will trap and hold moisture.
Spray in bed liner 1st then horse mat could be the solution?
I am looking into this product I just read about: Al's Liner http://www.alsliner.com/
"Al's Liner is the first and only urethane/acrylic DIY bed liner kit that delivers quality protection that meets or exceeds the so-called "Professional Bed Liners."

IMHO, you can never get rid of humidity or moisture, and you can never perfectly coat things to perfectly exclude moisture from getting in/under. In other words, it is wise to always expect that metal will see moisture that will cause rust.

I have found that "Cold Galvanizing" paint with at least 93% zinc will effectively galvanize steel and prevent rust; it does so even if there are small bare spots of steel as the adjacent zinc will prevent the bare section from rusting (at least for small distances, like a few millimeters). The difficulty stems from the need to strip down to bare steel, apply the Zinc based paint/primer, and then apply a topcoat to meet your needs. This will prevent the base metal from rusting even if it is submerged for long periods (or has water trapped against it from rubber mats, bed liners, etc.). This is effective until the Zinc is consumed and then the protection is lost, but I've not yet seen rust on the Zinc painted steel in my projects in over 6 years, without any additional protection.

Then you can put whatever coating or noise reduction materials you want over it.

But if you coat over a zinc spray, you aren't getting good adhesion for whatever liner or paint goes on top. Those sprays don't lay on like typical auto paint or primer.

This has been discussed a lot but in my opinion, based on witnessing some scary results, it's a bad idea to use spray-on liners in a Pinz because they will all promote rust eventually and you won't know it, often until too late. Some years later when you or the next owner need to do repairs you will have a nightmare on your hands trying to remove that stuff.

A heavy sound absorbing material laid over a zinc coated bed is the best solution in my opinion. The good stuff isnt cheap though. The two mats on the bottom here: http://www.heco.net/noise.html Also: http://www.cabinsulation.com/ You can remove them once a year, clean, re-coat etc.

A padded ceiling is a major way to cut it down since the sound waves are bouncing off it and making it much worse. That's why the trucks are so much nicer to drive with the tops off.

In my professional dealings with spray-on or roll-on "bedliner" products, I can tell you they do a great job of cutting down on some noise, but are also great at hiding rust.
With new trucks, I always tell people they are a great choice, but when people come in looking to protect the metal in their 1971 I-H Scout or their 1975 FJ40, I am hesitant. Just my 2 cents.