I have a Dell Optiplex 9020. When the computer enters sleep mode after 15 minutes, you can hear the fans continuously rev. They will speed up for about 3 seconds, slow down for a couple seconds, and then speed up again for a couple seconds. This process repeats continuously until the computer is woken back up.
I don't have any issues otherwise... computer seems to wake up fine, and the fan never kicks up to that speed while the computer is actually in use.