12.7 under extreme boost 28-30lbs I hear teapot sound. Air Dryer?.

Remember on those old engines the air compressor pulled from the pressurized intake stream. If you look at the diagrams for those old air dryers they have what I believe was called a turbo protection valve. Something in that vein anyways. That's what's bad.

Sorry I don't remember any more details, as it was 25 years ago last I messed with one

 

Picture of this tank would be helpful... I think I have an idea what the tank is but want to see before I say.

 

If you have a 12.7 with egr check you're rubber tubes going up to the hood. They can bust causing over boost situation. Also the charge air cooler may need checking.

 

So no EGR engine?

 

Does your rig hold air if the rig is shut off? Not too long ago my air dryer filter developed a sizable hole. I change my filter every year before the first cold snap. I thought it was odd to develop a hole but my mechanic said he sees it occasionally. That would make a large hissing noise at higher boost. I have a 12.7 with a nice tune and my turbo whistles going down the road anyway.