i've noticed that the sound is present when the unit is "heated up" like 5 minutes after u got her on...
maybe a small heatsink over the U25 can solve the problem? i will try that :D
edit after a teardown:
the processing unit it's not so heated like i supposed, but an lm317t burns like a hell... i suppose it's 70-80°C because i can't keep my finger on longer than half second... maybe that's the fault idk, the other lm317t aren't that warm