[Elecraft] OT power supply question

[wt5y at netzero.net]

I use a Pyramid power supply.  PS35G to be exact.  Sometimes when I turn it on it makes a thudding sound.  But it works fine and it doesnt do it all the time.  I am not the original owner and Ive had it for a couple years so I know its getting pretty old by now.  Wondering what would make that noise.  Kinda sounds like someone hit a metal box with a mallet best I can describe it.

WT5Y