Totally a fan. Open up the plastic cover and unplug them one at a time, one of them has just become loose and is vibrating. You'll have to replace but I think they're like $20. Basically its a computer fan. Sometimes if you turn them both around the noise will stop. One sucks in air and one pushes it out, try flipping them both, I did that for a while a couple of years ago. Unfortunatly the fans are pretty pourly made and you have to replace them from time to time...
Now I had one go 2 months ago and just unplugged it, I haven't had any heat issues without it running so I'm just gonna leave it.