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As I'm preparing to get started on my truck, I'd like to do some sound dampening while I'm at it. I'm curious if anyone else has done anything along these lines, too!

I'm planning to use a bunch of spray on bed liner. Im thinking that using it as an undercoating on the underside of the fenders, cab, and bed, as well as inside the cab can't hurt, and might prevent some damage from rocks/ road debris...and coincidentally, its also excellent for killing noise levels inside a vehicle, since it dampens the vibrations on metal panels.

I'm looking at adding something inside the doors to kill the vibrations there, too. Dynamat makes a good product, I'm told, and I know there are many alternatives out there. I haven't decided on insulation & carpet or thick rubber mats on the floor...

I'm not expecting my M37 to ever be actually quiet inside, but after spending 20+ years around military truck and equipment, I have tinnitus and hearing loss to show for it. I think taking the sharp edge off of the in-cab noise is a very good idea.

Thoughts?

I've had excellent results using Dynamat on other vehicles. Not only does it reduce noise, but it also insulates, keeping out some of the heat from the engine and drivetrain.

Just be sure to clean and prepare the surface properly, and to cut the pieces in a way to allow them to fit around curves and obstructions. I usually cut paper patterns and test fit those, then trace the finished patterns onto the Dynamat. Also, a good quality hard rubber roller helps to adhere the Dynamat and push it into tight spots.

I think Eastwood makes a similar product, but I've never used it, so can't say how it compares to Dynamat.

Since you are dampening the sound, there is NO way to soundproof, realize you do not need full coverage of the panel. A rectangle inside each door covering around 60% of the panel is more than sufficient to dampen any resonance and much of the transmitted noise. Several pieces along the back of the cab are all that are needed. I would do as much coverage on the firewall mostly for heat insulation. Don't forget to put 4 slabs on the underside of the hood. That is a huge noisemaker. Another advantage of pieces in the middle of panels is they don't trap water in the edges and seams to promote rust.
The undercoat will do a ton for noise reduction, just make sure you don't coat the fasteners if you ever want to service any of it.

I just got done doing what you are asking for
I use peel n stick from Lowes.
$30 buys two rolls.
Its a old hotrodder /custom stereo installer trick

I ranted in another thread about the evils of bedliner. Just beware that any panel repairs where the bedliner is will be difficult beyond belief. Dynamat or simlilar materials will deaden the sound better and will make repairs far easier.

I used the 4x8 3/4" rubber horse stall mats.
Cheap
Last forever
Great results

I actually have 3 layers
Peel n stick
Double foil sided bubble wrap
3/4" thick rubber mats
I don't have to raise my voice and am considering a stereo now