No worries about bringing up an old thread.

In fact, old threads are new threads to newer members.

You are correct, there isn't 8" of clearance to all engine components. The degas bottle, the top mounted alternator, the batteries, and the tap sleeve for the air intake on the driver's side valve cover are some of the high points of the engine, but not the hottest points. The hottest part of the top of the engine is the turbine housing, which is tucked under the cab forward cowling well away from the hood.

Perhaps the best underhood insulation for attenuating sound is the OEM hood insulation blanket that Ford fitted to the 2002-2003 Excursion with the 7.3L diesel engine. No other undherhood insulation blanket offered by Ford before or since has been thicker.

I replaced my stock 2000 underhood insulation with that of an Excursion (that I bought new from Ford parts in late 2002) due to the difference that I perceived between the two.

Dynamat type sheet membranes, whether butyl based or asphaltic based, will not successfully suppress the percussive type of sounds that the 7.3L diesel makes. One needs soft, absorbing, open cell materials to dissipate that sound energy.

The easiest way to hear what works to capture and kill sounds is to operate the truck over unpaved ground covered in a thick layer of tan bark or a thick pile of leaves, and then turn the truck back onto a hard paved surface again. The audible difference is instantaneous. The same principle can be applied to hard reflective surfaces of the underhood sheet metal. A thick layer of dense, but open cell materials (like the thermally rated and self -extinguishing semi flame suppressant material Ford uses) acts like a layer of leaves or tan bark, where sound energy enters the labyrinth of layers and loses energy with every directional change within the matting... as opposed to hitting one flat surface and bouncing off, echoing the sound again.

 

I thought I had a pic of the sound deadening on the firewall, but here's a start.

 

I have an L'99 and there's no way I want to tear my truck apart that deep just for a few decibels.

 

A lot of noise is also from the torque converter in automatics as they get older they break up inside and they get noisy. I had mine replaced and it made a huge difference in the amount of noise both inside and outside the cab. You can stand outside and talk in a normal voice with vehicle running. I never noticed it but a uncle is a ford trans guy and he took it in for some services and said my tc was making noise and he wanted to replace it. So I had him do it. He went through the whole truck from bumper to bumper. The only other thing it needed was brakes and rotors on the front.