Agreed. As compared to my 01 S4 (even in stock form), I've always thought the k16s were dead quiet.

For the dude that was saying he was hearing them at 4k when they are hitting - thats not the spool, that would be them bringing the hammer down...you would hear the spool at lower RPMs and its generally kind of a whistle or (as stands to reason) kinda like a turbine engine winding up for the pitch.

 

What you hear depends on your intake, DVs, exhaust... more than your turbos.