New Tenma 72-10495 Benchtop Supply - odd noise when turning on

[banedon]

Hi guys

This may seem like a really silly question, but should you expect an odd twang noise from inside the case when the unit is turned on. It sort of sounds like someone vibrating a ruler off the side of a desk. Or possibly like an old CRT monitor degausing. Doesn't always happen, though.
As you can tell, I've never owned a proper benchtop power supply.

[banedon]

Is it possibly dumping power from the PSU capacitors?  Seems to happen on the second or htird time you power the unit up if within a few seconds of being turned off.
So :
Turn it on - ok
Turn it off
Turn it on with a few second - odd noise
Turn it off
Turn it on with a few second - odd noise, but possibly more pronounced

[GreyWoolfe]

I have a couple of Astron 35A linear power supplies for ham radio equipment.  Both of them make a "twang" when turned on, whether or not there is a load attached.  Both work just fine.  I might check one on the DSO just to see what happens when the supply turns on.  Unfortunately, I have work related things to do before I can clear the workbench.

[mikerj]

It may have a PTC in series with the transformer primary to reduce the inrush current.  Switching it off and then back on again quickly will defeat the protection the PTC gives as it won't have had time to cool down sufficiently.  Inrush currents can be very high, and the strong magnetic fields produced can be enough to make the transformer noisy.