Hey DTS fanboy,
There is a detailed description of several methods to make the HD2 more silent over in the German section.viewtopic.php?f=56&t=13886
Basically, he describes 3 methods:
1) Half as loud
Open up the HD2, unscrew the fan. Put 4 small rubber gaskets between fan and board before putting it all back together. This made it a lot better, but the OP was still unsatisfied.
2) Absolutely silent
Completely remove the fan, leave one side of the HD2 open for cooling. The OP found no adverse effects after 8 days, HD did not get hot. Not the best way of course, as an open case doesn't look nice and might attract some dust.
3) Minimal noise
Replace the fan with a better, lower RPM fan. OP bought the fan and a set of silicon gaskets for 12 Euros. Fan needs a 2 pin connector however. As the screws of the new fan didn't fit he needed a drill to make the holes larger. This will of course definitely void your warranty...
Hope this helps. All credits to 123Teddy, the OP.