Undercoatings don't do much, and tar based damping products will suck if it gets hot. Recommend butyl rubber (I like Second Skin Audio). I've damped and insulated most of my Pathfinder and it's about Camry-quiet now. In order of biggest bang for the buck:
- Damping the front fenders. The body-on-frame construction means these are unstressed, and boom like a tympani drum over the smallest of bumps. Biggest improvement by far. Once you cover about 50% of the area (aim for the middle, and knock on the panels to see if you are killing resonance yet) it's going to be pretty quiet.
- Foam in the front fender wells. Hard to get an effective barrier with thicker products like Luxury Liner between the plastic fender well and the metal, but it's possible.
- Damping the roof. I actually haven't done this (the roof cover is pretty big and time-intensive to take out) but there's lots of thudding that comes through here. Again, big, unstressed metal panel (no sunroof on mine) and experience says it'll make things better. Also, you don't hear rain afterwards
- Sheets of Luxury Liner over the rear cargo area. I have had the entire interior taken out for damping, but there is effectively no way to get a good sheet of foam plus mass loading over it. So I got lazy and put two sheets of this stuff across the cargo area as kind of a mat. Kills a lot of road noise coming in from the floor. I usually have my rear seats completely folded for mountain bikes, so I have lighter sheets of neoprene foam that will go over the second row folded.
- Damping and an insulating blanket in the hood to replace the OEM insulator. Absorbs road noise and a lot of engine noise.
- Damping and foam in the rear hatch. Lots of road noise comes in through the back door. I no longer hear my bike rack clinking and whatnot.
- Damping, sealing, and foam in the doors. Not much noise reduction here, but noticeable improvement in stereo audio quality. Doors are heavy enough that it's not a crazy improvement like it was in my old Sentra.
- Damping rear floor and interior wheel wells, plus some foam. Not a lot of noise reduction honestly. The metal is pretty thick and already doesn't resonate much.