I have a 4 year old Systemax desktop.  For the past few months, upon startup the bottom fan (there are, two, right?) makes a strong buzzing noise on occasion.  If I whack it directly over the grill, it stops.  This is happening more frequently.  This morning, it kept resuming the noise after I whacked it. Finally was OK after awhile.  I possibly noticed that before whacking it, it was kind of warm...maybe it WASN't going, even though the noise was coming from it?  I was going to take off the case to see if there is something in there..but can anybody give me any hints?  And if I have to replace this, what do you call it?  For the heck of it I went to NewEgg and typed in "Computer Fan" and all the results seemed to be the upper one, not the lower one.
Thanks.
				
			Thanks.