I swear by
this Gold Thermal Barrier...
But like John316 warns - it's also expensive (most "good" thermal/noise control products are). If you are really worried about focused areas (like the firewall right next to the turbo, or the incoming fuel lines) a couple of square feet will be enough to wrap braided hoses, (etc) without breaking the bank.
If you can, you will get the best performance from a sandwich of materials - a "loaded" paintable skin sprayed on the metal inside the engine compartment (frame, skin - not the block!!), then a 1/4" to 1/2" layer of closed-cell foam, then a "loaded" layer of heavy vinyl or lead, then another layer of closed-cell foam or rock-wool (aka mineral-fiberglass) and finally the outer cover (some kind of heat reflecting barrier).
The outer barrier will reflect most of the radiated heat from the engine, the rock-wool will absorb some of the higher frequency noise from the engine (turbo whistle, valve rattle, fan whir) and slow sown the passage of thermal energy (insulation can't stop heat it only resists it), the lead sheet floating on the closed cell foam will absorb a lot of the airborne low-frequency noise (hum, combustion noise) by deforming upon collision with the sound-waves, the closed-cell foam should absorb a bit more of the low frequency sound energy and slow down a majority of the remaining heat energy that gets past the rock-wool (but some will still get through) - and the loaded paint will add a bit of mass to the sheet-metal/framing to reduce it's resonant frequency.
To explain that last part a bit more, if you tap a piece of metal with a coin - the sound you hear after the coin makes its initial impact, is the metal resonating until the impact energy added to the material dissipates. It will sound kind of like a “clink” that runs out for about a second or two – but with a pliable-load on the surface and the same impact, after the coin hit and makes its “tap” the resonance should sound more like a low “thump” and it should die out almost immediately (the loaded paint is a bit flexible and will deform as the metal vibrates, converting the mechanical energy into a little bit of heat).
In recording studios we do the same thing – but then in the sound-stage, we will even “float” the floors and walls (mechanically isolate the room from the structure). While you could do this in a bus, it’s probably impractical – but “floating” the engine and drive-train is not (these are usually vibration isolators that the engine/tranny are mounted to). Let me tell you, you haven’t driven a quiet vehicle until you’ve driven one with the drive-train properly isolated!!! (just think of all the things in your rig that wouldn’t be vibrating and adding to the perceived noise of the space)
You can pick up a little more sound isolation if you carefully separate all radiated sound energy from the passenger space (it has to go somewhere, so absorb it, diffuse it or direct it). For a “pusher” bus, adding a mud-flap that goes all the way across the back right about there the transmission meets the engine, and then down along the sides of the bus towards the back near the bumper will help to “direct” the sound energy out below the bumper (not forward under the bus towards the passengers, or out to the sides to bounce of buildings and passing cars/trucks back to the side windows). Adding the above mentioned sound treatments will help with absorption of sound energy, and adding structures in the engine compartment and under the bus that have pyramid shapes or random surface features will help to diffuse sound energy – much like light waves, when a sound wave hits a surface it either passes through (some of it can be absorbed, the amount depends on the type of treatment), and some of it reflects. Think of a mirror, a hard flat surface will reflect sound energy with enough uniformity to retain a lot of the fidelity of the sound (i.e. you can hear what the source sounded like through the reflection). And again, much like a rough/jagged surface (even if made of a hard “mirror-like” material) breaks-up a reflection, you can’t make out the image that was reflected as clearly since not all of the energy was reflected in the same direction (this is diffusion).
Hope this helps…
-Tim
You guys are NEVER going to let me live that down are you.... 
