[Cozy]

What you're getting isn't compressor surge but you are right, it sounds as if your sping is too stiff for the boost pressure that you are producing i.e. it requires higher pressure to move the spring than your engine can provide. This means that the air forced out by the turbo into the charge pipe/DV is then forced back into the compressor wheel when it hits the closed wall of the DV. The air has nowhere else to go so the chatter sound you hear is the air clipping the compressor wheel and slowing it down which in the long term is not ideal.

A lighter spring would be rated at a higher negative PSI however, you shouldn't really be experiencing any negetive boost in conditions other than idle or extremely light cruising at best. Normal driving and WOT runs should all be registering as positive pressure and therefore DV's are generally rated for positive pressure i.e. "Green spring 15psi" meaning that unless your turbo is able to produce 15psi+ of pressure the spring will not open at all. Is there a service kit for your DV or is your DV old or well used? Some manufacturers offer a small repair kit which comprises of lube and new seals to eliminate the DV sticking open or shut.

For reference compressor surge is on the inlet side of the turbo. This is when your turbo is sucking in more air than the engine or components there after can consume. It causes inlet air to 'bung up' in the intake system all the way back to the turbo and is idenfied as a chattering or chuffing sound under boost while essentially the compressor wheel just chops static air caught up in the turbo housing. This can reveal a bottle-neck or restriction in the intake system such as an IC that is too small, piping that is too small, a wholey inefficient or restrictive IC or ECU mapping that cannot handle the mods and needs adjusting. A good example of the latter is installing DP's onto a stock or tune only car for example, the turbo is allowed to move more freely and build more boost but the ECU really has no idea what to do with it as it expects you are stock or tune only. I've had this before and it revealed that my stock IC was just too small and that the mapping was no longer able to control things as it wanted i.e. it was not expecting to be running with hardware. Both things changed and the car was an absolute beast. Anyway, I doubt that you're having this issue, I haven't heard of any N54/N55 cars suffering but then again everyone on here seems to apply the correct tune almost right away as it's so easy with Procede, JB4 & COBB to flick maps rather than head back to that tuner so many miles away.