[nsx_23]

Too much coffee?

 

[olblueyez]

Its the built-in speaker guys!!! Geeeezzz

 

[scytheavatar]

Someone call the Justice League of America...

 

[pyrokid]

I'm not sure, but it could be capacitors inside it.
I don't know at all how amps work, but I know some about electronics, and caps make a mid-high pitched noise when charging/discharging sometimes.
(y'know, like the noise disposable cameras make when you charge the flash? )

 

[daglesj]

Could be something vibrating when hit with a constant tone type. Some sort of resonant frequency?

Heck stranger things have happend (picking up radio on braces?).

 

[Navyblue]

It's not a stretch at all if some components inside vibrates when power (audio signal) is applied, like the others I'd speculate it's the capacitor. Probably some PCB or capacitors is not physically secured as well as the others so it doesn't happen to all units.

 

[DynaFreq]

I would make a video normally to show you guys, but i don't really want to try and make the amp do it again.......