|I'd be very much inclined to replace it, and if under warranty - exercise that right.|
If there is a transformer in the psu it may be the laminations are in effect "rattling/vibrating" at whatever your mains frequency is. Equally you may have large smoothing capacitor a little under stress, not happy. Either way "something" is getting a little warm - which is why (possibly) it stops(?) when the cooling fan operates.
I suggest it would be wise(r) to replace it - whilst under warranty. It does sound a little like all is not well with it, maybe not upto par.
message edited by trvlr