PC desktop making noise after waken from sleep mode?

So I recently installed a new 650w PSU from a 250w and installed the NVIDIA GeForce 8800 GTX video card into my PC desktop. However, about a day or two later, when I put it on sleep mode and wake it up later, the very back of the desktop case (the silver-colored panel where all of the cables are plugged into) starts making a very faint rattle-like noise. It's pretty quiet, but my best description of the noise is as if a small fan is hitting a tiny wire. Originally I thought it was the new PSU, but I then later realized it was the back of the case, since, when I push it inward with my fingers, the noise stops. It's very strange, especially since it only happens when I wake the PC from sleep mode (my OS is Windows 7 Home Premium 64-bit). The sound goes away after about two minutes or so. Can anyone guess what is going on?