Odd noise coming from the horizontally placed fans

[zalbard]

Houston, we have a problem.
I have quite a few fans in my second rig, and only two of them are horizontal.
At first the one at the top of the case started making a weird sound sometimes. Hard to describe it. Reminds me of the pump noise a bit. I could stop it, then let it run again, and it would be silent. I got tired of this and removed it.
Now the same started happening with the second fan. The problem is that this second horizontally placed fan is on my graphics card, and I cannot really replace it.
Is there any idea what I can do to fix it?
Any help would be much appreciated. Thanks!