Re: Screaming Noise of Death: air brake issue?
I highly doubt the noise is air brake related. The compressor is unloaded, just the same as say an AC compressor is, until it is needed. In the case of an AC compressor a clutch is used to disengage the engine from the drive belt. With an air compressor the "uncoupling" is done via an unloader valve (there are many different types) which shuts off air flow to the tanks and removes load from the compressor, typically by allowing the two cylinders of a twin compressor to pump back and forth, or by keeping the exhaust port of the compressor closed so that most of the energy used to compress the air is returned to the system by the expansion of it within the cylinder, not dissimilar to the system GM uses to reduce pumping losses on their displacement on demand systems.
Now...that's not to say it might not be a compressor, but it would have to be loaded and you'd be able to replicate that at a standstill and you probably wouldn't always get the noise while moving.
Please don't be offended by this statement, but I think you're probably a little fuzzy on the mechanics of air brakes. It's an incredibly simple system, but it's important to know how it works before operating the vehicle so please pick up a CDL manual from the DMV as it will have a fantastic air brake section in it. Heck, wikipedia and howstuffworks are also great places to look.
Now then...back to the noise. Your description is really throwing me. I just can't fathom what that would be. Is it engine speed or road speed dependent? Easing off the gas, unfortunately, reduces both, as well as load. Take the bus up to the point that it makes the noise (I'm assuming it must start below say 45-50mph?). Shift the bus into neutral. Your road speed will be consistent, as will driveshaft speed (for the most part...don't do it uphill), but your engine speed will drop off to idle. Does the sound change? If it does, you can verify that it is a speed/load related noise by downshifting from 4th to 3rd (make sure 3rd gear will not overspeed the engine) and see if the intensity changes (it should get louder or higher in frequency).
I think it's probably turbocharger related like a blown off or leaking boost hose. You can't replicated it at a standstill because (as an engineer should know) turbos work off heat in the exhaust and you just can't get higher EGT's without a load on the engine.