The wind up comes from the motor, the gears and the "wind up" of the compressing the piston. Sort of a ziiip-twack of a shot. Plus the motor noise.
A precocking gun, that cocks the piston at the end of the shot has a "backwards" wind up...sort of twack-ziiip where you hear the shot, then a wind up, plus the motor noise. HK33E, PSG1, G&G L85
Some guns basically eliminate the delay (or comes very close to it). A working PTW has near instantaneous shot. Any "wind-up" is minimal compared to an AEG. So basically you just get a twack sound from each shot. No recoil or any "rifle-like" effect....but no sewing machine noise. Mind you...you'll might still hear the motor...but not the gears when the piston is being compressed. If you haven't tried a PTW, try one if you can. When they work, they're gold...when they don't they suck.
Any of the "blow back" AEGs are either destructive...or are really dinky. More of an annoying clink-clack than actual bolt/firing noise. Sounds like your bolt is made out of a pop can when it cycles.