because that is how they work. the AC is always humming, and "Switching" conversions and regulations Buzz & hiss. the more power you use the more you will hear it. assuming you can hear anything over the roar of fans. Meaning they are always Doing switching.
if parts are not properly clamped down or comming lose this noise could get louder
if things are dying inside internally they could get louder then give out
if the thing is being pushed to the limit all the time, it is more likely to fail over time.
if certain parts are failing , other parts can be worked more, like if a cap is failing a torrid choke it was cleaning up after will make more noise, stuff like that.
Many GPU conversion and regulation curcuits Hiss from the digitial switching power regulations too, usually it is when it is loaded, and you have to be able to hear it over thier obnoxious fans.
on one GPU board i got the fans quiet by adapting in mondo fans, then i get to hear the hissing power curcuits instead.
Non-digital switching ones didnt have the noise, and some digital switching ones they cleaned up the noise using various tricks, like sealing coils, or using different/more parts.
I would worry about it WHEN:
it gets louder than it originally did when using the SAME ammount of power that you had before.
If i suddenly heard it and it never made that sound before , and i had checked
If a Capacitor is bulging or the top is splitting or in any way looking defective
If when you check the Power comming out it is wrong, or dirty
If many other users report that it Fails, but do Not report that it makes noise
if this is the first time you used that much power, it might be normal but you might be pushing it too.
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