GX620 makes noise before turned on

My company just bought 6 Optiplex GX620 PCs.  They are all the same except that two of them have DVD-RW drives instead of just the DVD drive.  When I plug in the power cord it causes the PC to "rev up" briefly (but loudly).  I think it's the cpu fan but I'm not 100% sure.  It also does the rev up thing when I press the power button.  The rest of the time the computer is very quiet. Another strange thing is that this behavior does not happen on the two computers that have DVD-RW drives.  I've never had a computer that made noise when it wasn't even powered on.  Has anyone else experienced this?  I'm a bit worried I might have gotten a batch of lemons.