Rattling PSU fan

[ShellShock]

I have just put my old PSU (OCZ600MXSP) into a new case (Bitfenix Nova Midi). The PSU fan is now rattling when the case is vertical (PSU fan pointing down) but is quiet when the case is horizontal (PSU fan on its side). In my old case the PSU was OK, but it was at the top of the case with the fan pointing down into the case. In the new case the PSU is at the bottom of the case with the fan pointing down against a grill in the case bottom (there is also a grill on the bottom of PSU covering the fan).

I am not sure whether the PSU is being squeezed a bit so that something is touching the fan. But is this was true would it not rattle when the case was on its side?

So is the PSU failing? It has already survived a mains power surge that killed my old motherboard/CPU. It is currently powering my new case and build OK (Windows boots).

 

[ShellShock]

I think I have solved the problem. I unscrewed the PSU and lifted it up--there was a weird little plastic sheet trapped underneath it (some packaging perhaps?). Anyway, it is now silent.