Antec 900 pulsating hum?

[PNemesis]

Can someone help me solve this?

I have Antec 900 and it's making hum noise but pulsating hum noise, like every 2 sec it goes "mmmmmmm" that last for 2-3 seconds. All the fans are low, but this is not the fan issue, 'cause I unplugged all fans and blocked CPU and GPU fan to eliminate possible threat.

Is it possible due to 3 hardrives that I have in lower cage?

This happens constantly, if computer runs something or it is in idle mode.

This is the pic of an empty case (taken from internet) but I marked where components are:

Thnx.

 

[Onus]

It could be a hard drive motor. Power the machine up with only the boot drive plugged in, and see if you get the same noise. Make sure your drives are securely mounted. If mounted with silicon grommets, the hard drives should not be screwed in so tightly that the grommets aren't able to absorb vibration.