PSU problem help needed

[Barett]

Hi

I have a Watson W-30am 30-35 Amp peak power supply.

After using an IC-706 running 30-40 Watts on PSK31 for a few hours the power
supply starts making a very load buzz/hum on keying the radio. The noise
decrease's and increases turning the power up and down on the radio itself.

I have also notice the buzz/hum is still there when the radio is
disconnected and a small load say 1 Amp connected to the PSU, although the
noise is a lot quieter. So I don't think that the radio causing it.

When I start it all up the next morning the power supply is running fine
again, no noise until I have been keying the radio in PSK31 mode for several
hours when the very load concerning buzz/hum starts again.

The power supply has a top case fan that draws the heat out of the case and
the power supply case doesn't feel hot to the touch. When the load noise
occurs

Does anyone have any thoughts what could be causing this?

Thanks for any advise or help in advance.