APC UPS Transformer humming

[J.harris]

I have a 1500VAC APC UPS that is about 6 years old.  I acquired this unit because it makes an annoying loud humming sound as it warms up and the owner no longer wanted it.  I determined the sound is coming from the transformer and silences with pressure being applied to it.  The transformer originally didn't have bolts installed holding the plates? (not sure of the correct terminology) together, so I installed some and the noise is down considerably. 

The noise is very tolerable now, but cutting out the hum altogether would be wonderful.  Does anyone have any advice on how to make this any better?

[SeanB]

A steel angle to apply force to the laminations held by the bolts will help more, with a thin sheet of bakelite between the angle and the laminates as well to reduce eddy currents as well. If you do not insulate the plate it will make a shorted turn and damage the transformer. Insulate the bolts from the angle though with fibre washers and a sleeve in the hole. A thin sheet of silicone rubber on the mounting plates to provide sound isolation will help as well, as it will provise extra sound damping.

[geraldjhg]

and put it far away
or pot it in tar

[Psi]

A while back i had a UPS that started making a rather loud and strange humming sound when running on battery.
It turned out to be 1 of the mosfets failing, the switching waveform wasn't right and that was causing the sound in the transformer.

If the hum your hearing doesn't sound like general 50hz hum then it could be something similar.
The hum i got really didn't sound good, so it was pretty obvious something wasn't right.

The UPS still worked though, but i suspect it wouldnt have handled full load.
It was under warranty so i didn't bother trying to fix it, just sent it back.