Remove the fan completly and run it with one fan if you don't hear the noise any longer then try putting the other fan in there and the 140mm in the front (if it fits like that). What kind of overclock do you have going and what is your cpu? While you have the fan out examine it to see if there are any visible reasons for the noise and even try running it not connected to the heatsink and see if you can see any reason for the noise.
I already confirmed that it's the 140mm fan that's making the noise and it's coming from the fan itself, no loose wires and stuff.
it seems the fan itself might be loose because after I squish its middle part (pushing its from and back part towards the middle), it becomes silent for about an hour of running it and the problem returns eventually.
It doesn't matter whether I overclock or not. The moment it turns on, it's noisy (unless I apply the temporary fix by squishing it)
The reason I asked if your overclocking is to see if you can run the cooler without the 140mm fan or if you need it because your overclocking and generating a lot of heat that needs taking care of. You'll either have to RMA the fan if it's still under warranty or buy a new one.
If it seems like the center part of the fan is seperating and causing the noise the objective is to keep it "squished together" and that's hard to do with a spinning fan.
A coup[le of ghetto ideas would be to 1. Tka a piece of duct tape and tear off a thin piece that you could wrap around the middle peice of the fan in between the blades. I don't think that this will be easy to do and I have never tried this. 2. If the tape doesn't work then maybe a wiretie would with the fat knob part being in between the blades so the fan will fit back in between the fins.