surge

[Scruggs Family]

 Rocky, I'm going to guess that the rumble you hear is your
 exhaust system just barely vibrating against the body of the car.  More
 boost means more torque which means more deflection of the engine mounts
(in
 reaction to that torque) and more movement of the exhaust system and more
 opportunity to encounter something it would usually not encounter.

 Surge is when the air flow goes through a rotating
 compressor in the wrong direction.  In the case of our single-stage
 centrifugal compressors this comes closest to happening when you are
running
 at large boost and the throttle is suddenly closed.  The resulting pressure
 spike can reflect back through the system and end up back at the compressor
 outlet.  With the throttle closed there is essentially no air passing
 through the engine... at least far less than there was a second before...
 and the driven side of the turbo, the turbine, is now essentially
 non-powered.  The spike which has now traveled backwards to the compressor
 has more energy than what is now coming out of the compressor and the flow
 through the compressor can for a short period of time can stop or even
 reverse... a condition known as surge.

 One of the biggest reasons why your engine has a bypass
 valve is to stop this from happening.  When the throttle plate snaps shut
the bypass valve opens and
 the pressure wave is deflected/channeled to... ideally... the intercooler
inlet where it
 keeps the flow going in the correct direction and the spike dissipates in
that loop.  This not only keeps the
 intake system airflow going in the correct direction for boost to again
 build quickly but it keeps the turbo from experiencing the radical change
of
 direction of internal pressures.  It is the bypass valve to which your VW
 friends are likely referring as often wearing out.

 Axial flow jet engines will occasionally surge when the
 conditions are correct... or is that 'incorrect.'  Sitting on the runway
 waiting for takeoff particularly if there is a strong crosswind, the upwind
 engine can approach surge as the pilot spools up the engine for more power.
 The air entering the engine from off-axis can cause what is called a
 compressor stall.  If that stall condition ripples through several stages
of
 the compressor... there can be a dozen or more stages in a modern axial
flow
 engine... then the normally increasing pressure gradient is interrupted and
 the high pressure in the combustor section will cause exhaust to come out
 the front of the engine as well as the rear.  Compressor blades are
designed
 to be stressed in only one direction and when they are suddenly bent in the
 other they have been known to break thus reeking havoc on the structural
 integrity of the rest of the engine.  Compressor stall and engine surge is
far less common on current generations of aircraft turbines
 as the art of compressor design has resulted in far more robust engines
with
 higher stall margins than even a generation ago.

 Regards, Gross Scruggs