I found the noisiest part of the unit was the dual PSU's - I've swapped one of thems fans out with 60mm Akasa Ambers and 80mm YS Tech silents, it made a big difference.
I've tried disconnecting the PCI blower fan and the front chassis 120mm intake and the noise they make is not *that* loud compared to the 10k's, the drives are def the second loudest part of the system after the standard psu fans.
I've now moved the server down to the far end of the loft with some rubber padding under the feet and it's a lot better.
Think I'll keep an eye on an auction site for some 7200 rpm ones and see what they are like all the same though.