TKD2

A psu has no moving components except for the fans so i would say it's somewhat unlikely that it could be anything else. There is a small chance that what you could be hearing is static elecrtical discharge from a capacitor - but since you don't mention any other problems i'd would personally rule that out.

As for opening up a PSU, that kinda depends on your confidence with such things. They do carry a lot of charge for a long time after they are switched off and i can tell you from personal experience they really can shock you
The best way to do it is to leave it earthed by leaving the power cord plugged into the mains, that works in the UK since we can turn our mains outlets off. If you're from a country which doesn't have this luxury don't even consider it, to put it bluntly.. you might die
I suppose the least risky way of cleaning out the interior is to buy a can of compressed air and secure the fans whilst blowing out the dust.