Being a Nissan driver (tut,tut!) I always get people asking about the noise my car makes from having no bov and asking if its dangerous.

Here are 2 vids I've found. The second one Is quite informative.

This is not dangerous:
SR20DET Compressor Surge - YouTube (off throttle compressor surge / flutter)
This is:
True Compressor Surge - YouTube (true compressor surge)