Go to a place that sells RV's. Go to the parts department. Buy a tube of caulk that's used for the windshields of $500,000 Class A's. Take the wipers off, and take the metal screen off of the truck. Re-caulk the body seams inside the cowl box on a day with no chance of rain.
Just about any Butyl Rubber Sealant will do. Butyl rubber stays flexible for decades so it doesn't crack or tear like a latex calk. When I do anything weather expose to a structure or vehicle, I think about how the rain can get in, seal the drip and splash points and provide a surface tension break for spaces where capillary action can suck water into a space (between materials or walls). GM didn't do a great job thinking about this on the "new-look" and "square-body" trucks. Why would they put a body panel joint in the bottom corners of the mostly square shaped windshield? By the way, square joints are the hardest type to seal, which is why hydraulic cylinders are round at the seal
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Leaks aren't a big problem unless there was nowhere for the water to go, or no opportunity for the water to dry/evaporate. If you use foam, make sure it's closed-cell type so that it doesn't suck in water and hold it. If you expect water is going to get under your carpet or padding, give it somewhere to drain or allow/force air to circulate and it'll dry out.
It's not just the mass of an object that determines its sound attenuation capabilities, it's the characteristics of the structure it is a component of. It takes energy to make and object move, increasing its mass increases the amount of energy required to move the object a given distance. If the object is decoupled from a structure (allowed to move independent of the structure), the energy imparted to that object will affect the object only as it is allowed to move separately from the rest of its structure.
Think of this now in a vehicle - what assemblies have devices which decouple an object from the rest of the structure?
- The most obvious is the suspension. If the axles were hard-bolted straight to the frame, you'd feel every pebble you ran over and every bump would be that much worse.
- Engine has engine mounts as well as hoses and wires which are flexible so that they decouple the movement of the engine from the chassis. If you replaced the soft engine mounts with hard metal brackets, all of the vibrations created by the engine would end up in the rest of the chassis.
- Body has body mounts as well as again hoses and wire which are flexible, and mechanical joints on steering and shift linkages which allow decoupling of the body movement from the frame. If you replace the bode mounts with hard welds, the whole body will contain the vibrations that the frame had
- Seat has internal suspension which keeps most of what is left in body vibrations out of your butt.
What an underhood blanket is doing to kill sound is becoming the first thing that a soundwave has to move in order to be transmitted into a body panel - if it is decoupled from the body the energy that the blanket absorbs goes into only moving the blanket and not the body panel, so no (or maybe just less) sound energy vibrates the body panel. That's less energy further down the transmission path on its way to your body and ears to feel.
Now think about the body panel - does it generate noise? No, but it is moved by noise generated from something else. So what is the bedliner doing? It's adding mass to the panel, which increases the energy required to move it. One's expectation should not be that a dynamat or bedliner application is going to kill all of the noise as it is very tightly coupled with the body panel itself if the panel moves the dynamat/bedliner will move with it - there will be something with enough acoustic energy (mechanical structure-borne noise or air vibrations as sound), to move that body panel. The dynamat and bedliner don't prevent the transmission of sound, isolators/dampeners do.
By having a carpet on a foam which decouples the carpet from the floorboard, the carpet can move independently from the floor board so if the floorboard moves it doesn't necessarily move the carpet layer. Under the hood a blanket might move but it doesn't necessarily move the body panel behind it. This is why decoupling is good for preventing acoustic energy TRANSMISSION. It should be said that if you are in the space where sound is, there are no barriers which would make dampening effective at reducing noise - in that cases absorption ("eating" energy) to prevent any reflection, and diffraction/diffusing or breaking up a reflection into many reflections, makes the reflection less harmful to what you want to hear.
This is one of the reasons I advocate for first finding out what sources of objectionable noise you are dealing with are, then attempting to kill the noise as close to the source as possible where it's most practical and cost effective - and likewise, you don't want to harm the "good sound" (like from your stereo) by over-treating the interior. Like was said in
War Games "The only way to win [Global Thermonuclear War], is not to play." Making sure that there is no noise to get to a body panel is what keeps you from having that fight at the body panel - keep the bad sound out of the inside of your truck or from getting to the walls of your truck, and you'll have less to do inside the truck for stereo audio to be of good quality.