Peter,

You are correct. The amount of minimum load depends on the regulator topology. There are Forward Converters, Buck Regulators, Boost Regulators, traditional linear, low drop-out linears, those using Synchronous Rectification, etc

Each have different loop responses based on the compensation circuitry, magnetizing inductance, amount of input and output bulk capacitance, current mode control loop or voltage mode control loop, etc.

So, the topology makes a difference in what is required as a minimum load. Some require less, some require more. Typical Forward Converters are regulating larger loads and might need the full 20%. I have also seen Buck Regulators require as low as 5% and up to 25%. It all depends. Usually, the 20% makes most of them behave.

The whine is typical of an underloaded supply as the control loop is running at a much lower frequency than the switching frequency. Sometimes that is also the sign of an improperly "gapped" magnetics design or poor construction of the magnetics (Switching transformer and smoothing inductor(s) if used). The gapping is used to reduce the risk of saturating the core.

In a minimally loaded supply...

The control loop is sampling the output voltage, comparing it to a known reference. If the voltage is correct, the correction is not required for long periods of time. Either the pulse width is too narrow if it is a Pulse Width Modulated (PWM) or if it is a pulse position type (inserts a pulse when the energy is needed), there is no correction required. The control loop is at a much lower frequency. The transformer is built to be a higher frequency device (targeted at the switching frequency) and doesn't know what to do with the lower frequency demand. If the bulk input capacitance is minimal (cost and weight savings), the energy is not there to supply the lower frequency. So, it gets it from the power source (line voltage) and can actually reflect the low frequency back into the power source. This can sometimes cause EMC problems with other things attached to the same Power Mains as conducted emissions and high frequency flicker.

I know, I know, too much information...

Ross