I just coated the floors of my M1008's cab with about 1/8" of Durabak bedliner and it lowered the noise level in the cab cosiderably. i also sealed some holes in the firewall that used to have wiring. Will underhood insulation lower the noise level in my cab at all? does it actually absorb sound or just not let it pass through? Also is there a reason that the exaust manifolds on this 6.2 use springs to hold the pipes on, instead of just a nut? Mine are too rusty to tighten without breaking off I belive, and they're leaking. Would like to carry on a conversation at highway speed once and a while. Thanks in advance.