Humming noise dissapears when switching between OS and Fullscreen Apps

[Raptor]

This one's going to be very vague and I honestly wouldn't know what information to give. I'm hoping someone else will have asked himself the same question and found an answer or knows a bit more;

When I switch between Windows and any full screen application the humming noise ceases and starts again as soon as either pop up.

I only have two fans installed, a CPU fan (Which does not make any noise) and a PSU fan (Which seems to generate this hum). It's a Zalman 460 Watt 'ultra-silent' so I'm guessing it has to do something with it detecting a change in power requirements or heat build-up and that causes it to cease for a moment and then switch back on (The hum, that is..) so I'm guessing it is the fan.

The other two things that could make noise are two 7200RPM HDD's that are about 1 1/2 years of age. They're silenced by using a Zalman HDD cooler, though.

So, any thoughts?

[Carbon Dudeoxide]

Any thoughts.....well....

Put your ear next to each fan/hdd and see what's causing the noise? Then post back what is causing the noise and we might be able to give ways to silence it.

[Raptor]

Like I said, it is the PSU fan. I don't need any ways to silence my machine, I'm curious if anyone happens to know what could make the humming noise cease while switching between full-screen applications and Windows...

[GX1_Man]

This is a job for Fed, the electrician. 

[Joe_Flanagan]

Do you have Q-Fan or not-so-Smart Fan and if so, have you tried disabling it ?

[Raptor]

Like I said, it's my PSU which acts on temperature change. I was only curious what could cause it to suddenly drop the noise when switching between applications. It is not a problem because if it was, it'd be solved by now, but nevermind.