I found this.... This explains it quite well... Now that there's a whole intake system full of compressed air with nowhere to go, and the compressor is not being driven by the turbine - so it can't hold the same air pressure that it did while spinning flat out. The air will start to flow the other way, which means going backwards through the still spinning compressor. This is where compressor surge comes into play. The compressor is still trying to pump air, but at the speed it's going it can't provide as much pressure as there already is in the pipe. This causes the compressor to "stall". Stall is a misleading turn used here, because it doesn't mean stall as in to stop spinning. It's the aerodynamic stall, like a plane that's tried to climb too steeply, or has slowed down too much to maintain lift. The smooth air flow through the compressor blades is broken up, and some of the air will flow back out through the compressor, making a hissing noise. Centrifugal compressors work on a "squared" relationship for speed vs flow / pressure, so if the turbo slows down to half speed it will only be able to hold a quarter of the boost pressure. The "chopping" or "fluttering" sound is caused by the accoustic effects in the intake system, determined by the size and length of the piping / cooler. Rather than air constantly flowing back out of the turbo it tends to come out in bursts, triggered by pressure waves in the piping hitting the compressor. Generally though the flutter is the sound of the pressure wave cause by the throttle closing bouncing back and forth between the turbo and the throttle plate (the longer the pipes the slower the flutter). Every time the wave hits the turbo it causes the compressor to cavitate, which makes the noise. It's not the blades chopping the air making the flutter. Since the blades are spinning at 80000+rpm you wouldn't be able to hear the individual "chops" If you listen closely you'll actually hear a more constant hiss sound at higher boost levels and rpm when backing off. (ie, a "whooosh" rather than a "chop chop chop") The loudest flutter tends to occur at lower rpm and boost levels of only slightly above atmospheric pressure (0psi). This is partly because the sound is "chopped" up more noticeably, and partly because the rest of the car is making less noise at lower speeds. This is also where it's least damaging. Car manufacturers don't like weird noises from the intake system, so they use restrictive airboxes designed to muffle the sound, and more recently (SR20 onwards) used a Blow Off Valve. If you wanna read the rest, go here: http://www.lexusownersclub.co.uk/forum/ind...showtopic=29047