On the contrary, I could believe that it's normal behavior for the X750 or at least in the range of tolerance. If it follows similar behavior to the X650 (it should, since it's the same original X Series design), then the noise it produces is like this:
Note that that noise is measured in an anechoic chamber with elevated ambient temperatures. Seasonic X and Corsair AX will indeed start ramping up under heavy load. You could well be going past 400W in a heavy gaming load with those parts.
Most people have experience with these PSUs running in a case with a bottom-mounted PSU with fan taking air from outside the case. In those situations, most PSUs including these are quieter at higher loads. If you're using the P160, it only has a single fan and has a top-mounted PSU, right? So the PSU would be subjected to ambients more like the hot box and less like the cold testing.
As suggested in the link, Enermax Pro/Modu87+ (Lepa Gxxx) is noted for having a very quiet fan, even quieter. If my conjecture is on track though, you're maybe as well off getting like a Fractal R3 instead and having your current X750 isolated and drawing air from outside the case. That'd be a cheaper upgrade than a new PSU.
Or it really well could be an issue with your particular sample's fan.Edited by mikeaj - 5/20/11 at 12:45pm