It could be back pressure, but less likely since it is in a room versus in a cabinet. It is worth a try to move it out some by extending the plumbing. However, the rattling you hear is most likely not from back pressure. It is probably the the result of damage during shipping. In my earlier post I mentioned I had to send a replacement chiller back after being damaged by the shipper. The very loud rattling noise when the chiller is in an active cooling mode is exactly what I experienced. When I asked PCI if this was something I could repair, they said not likely and I would be better off just sending it back for a new one. More than likely you will have to send it back, but I hope I am wrong.
I also notice it is in a laundry room, which will have traffic other than you (I am assuming). Point is, I hope everyone that may be in there doing laundry will be as careful around it as you so that nothing gets accidentally bumped loose causing a leak.
Sorry but I hope it works out for you. They really are good chillers though.