Plenty of people with the same issue in this thread. It happens at certain RPMs (IIRC, it's around 1,000 or something?).
I just have mine spinning slowly (600-900 RPM). There's a really steep ramp up if temps get out of hand, to ensure the system doesn't overheat. But so far I've only managed to trigger that in benchmarks.
Good to know. Im going to go with 1300rpm noctuas on the top then. 1600rpm 92mm fan on the rear and 2x 1700 rpm noctua on the side should provide a good balance on the 2 in vs 3 out and sort out the whiring at the same time.