[Vroom]

The drone sounds like a drone of some sort, when the engine is warm, and I'm shifting 4th to 5th, and back, mostly in the gear, not neutral. We discovered it also happened when letting off the gas pedal quick and de celerating.
His opinion is that it's coming from one of the exhaust pipes from the turbo. That the interior of one of the pipes might be going bad. He has checked everything else, bearings, and so forth. Turbo is OK.
What else can crack and make noise as the engine gets to normal operating temp?