Old thread but I thought I'd add some info.
If you have a Progressive Dynamics PD4590 like me, I got some info from their support.
They said their system can sometimes make noise from a few places:
1. fans (Does the noise ever happen when fans are off or in cold weather? Mine does, so not the fans.)
2. In the photo below, there is a round white part at the bottom with copper wrapping around/through it. If you use something plastic to reach back and put pressure on that part, the noise may stop. (Don't use anything metal to do this!) If it does, they suggest slightly bending that part one way or another or just slightly twisting it can make the sound may stop (be very gentle, you don't want to break that part off the pcboard).
3. Less likely is the round components at the front.
They have a refurbishment program where they will ship a refurbished unit with a 1 yr warranty and you return your old unit. It's $126 to do (as of April 2017).