It's either turbo compressor surge, or turbo compressor surge in a system w/ a blow-off valve set to contain notably higher boost pressure than was achieved when he let off.
When looking at turbo compressor maps, the line/limit on the left side is the surge line. The compressor can't operate at the boost level/mass airflow (combination of) conditions to the left of the surge line & becomes unstable. It's very hard on turbo thrust (& to a lesser extent) journal bearings.
On high boost systems using a blow-off valve set to contain say 30+ psi of boost, they often don't blow open cleanly if the engine's just run up to a fraction of the blow-off valve's set point. On a turbo gasser (w/ throttle blade), run up to 30 psi in 1st gear - turbo spooled, let off throttle to shift - throttle blades go shut, turbo's still spooled...airflow can't go anywhere, pressure begins to go over 30 psi & the blow-off valve pops open & vents the airflow so the compressor wheel doesn't suddenly see much higher pressures & stall/cavitate.
On the same setup more casually run up to just 15 psi, upon shifting/throttle blades shut - the turbo isn't spooled as fast. So the BOV opens more lazily/slowly, but often shuts/opens - oscillating or chattering.
