AKA as "Capacitor Squeal or excessive loading. You're pushing the 'board too far. When you back off the O/C the noise goes away.
MICROPHONICS
Semiconductors are not prone to microphonics themselves, but some other circuit elements can cause them. One case is the inductor used in some LC oscillators, particularly in the radio-frequency range. This effect can sometimes be minimized by firmly anchoring each turn of the coil with special adhesives such as RTV. Phase-locked loop circuits can be sensitive to microphonics generated by the physical movement of the inductor used in the oscillator.
Capacitor Failure Patterns in DC Circuits
The majority of capacitor failures, perhaps 90%, involve the internal shorting of the capacitor. Once shorted, the capacitor frequently causes power-supply overload, often causing the power supply to blow a fuse or other wise fail. In less drastic failures, the capaitor no longer functions to isolate DC voltages from one circuit to another. If power is still applied, this failure is easilly determined with a DC voltmeter since there is a zero voltage drop across a shorted coupling capacitor.
...If the electrolytics are bad, the oscillation can be a squeal...
James Perozzo: Practical Electronics - Troubleshooting
http://repairfaq.ece.drexel.edu/sam/smpsfaq.htm#smpsstsm
Sounds that SMPSs make
Most switchmode power supplies when operating normally produce little or no detectable sound. The switching frequencies are usually well above the range of human hearing, but your dog or pet dolphin might be driven nuts!
However, under various fault conditions, and sometimes when lightly loaded, there may be tell-tail audible indications of the SMPS's state of happiness. The cause may be in the SMPS itself or its load.
1. Tweet-tweet-tweet or chirp-chirp-chirp (sometimes flub-flub-flub) - Short circuit or current overload. This is usually an indication of a shorted secondary-side rectifier and/or if in a TV or monitor using an SMPS, a shorted horizontal output transistor. The power supply is in a repeating cycle attempting to start up, being dragged down by the overload, and shutting down.
2. High pitched, but audible, whine - Excessive load. Like (1), this may be caused by shorted components. For example, a common failure a Panasonic VCR power supply is for the 18 V zener diode across the 15 V output to short due to dried up electrolytic capacitors. The result in an overload and whine, but usu
