[Al Skierkiewicz]

Boost Buck supplies use high frequency switching voltage into an inductor or transformer. If the frequency should get low enough, sub harmonics of the switching frequency become audible. In some cases, the core of the inductor or transformer actually vibrate and you may be hearing that as well. Most transformers and inductors are manuufactured with a little glue or varnish to keep this to a minimum. The hard knocks of competition may break this bond. A search of this forum will give you some other references to the noise from the design team.