As impressed as I was with the improved DVD picture quality and video processing capabilities of the HTPC, I was equally amazed at how loud it was. I am used to sitting next to a PC in a noisy office where the noise just becomes part of the background din, but in a quite room it was LOUD! I measured the noise from the original configuration to be 45dB at 2' (my SPL meter doesn't go low enough to measure at listening area but I promise, you could hear it loud and clear). This was gonna change...
The worst culprit was the CPU fan, by far. I was using a stock Coolermaster with Artic Silver II heat sink compound. Seemed like the power supply fan was in second, followed by the case fan.
First I bought some thin Frost King brand thin vinyl foam stick on duct insulation and lined the case with it. Not much help. Not much increase in temperature either. Processor was running at a quite cool 39C.
Next I cut out the stock punched metal fan grilles, not much benefit here.
Changed the Coolermaster to a Thermaltake Orb cooler, rated at 29dB. This helped some but I don't think that it ever operated anywhere close to 29dB. It also did not cool as well, core temp was now running 45C.
The next step, and most dramatic, was to put more insulation in the case and build a "backpack" type hushbox for the PC. See the pictures. This box cuts down on the noise that escapes through the ventilation holes in the case, while the quieter fan at the top facilitates a chimney effect airflow pattern through the box to keep the hot exhaust air from backing up into the case. It is made from 1/8" acrylic sheeting with clear Liquid Nails and clear insulation tape at the corners. It is friction fit to the back of the HTPC with black 3/8" wide vinyl foam weather stripping.
I also put voltage dividing resistors on the case and power supply fans plus the CPU cooler.
This helped quite a bit, while still far from silent, I could no longer measure the SPL with my meter at 2'. A further enhancement was to change back to the original Coolermaster, which ran quieter and cooler than the Thermaltake with the voltage dividing resistor. CPU temps now run 48C to 50C with case temps in the 34C-36C range.
I also started running a DVD ROM drive speed controller to quiet the Pioneer 106S. This helped quite a bit, especially when starting up DVD's or CD's.
I think that to make more meaningful reductions will require some wholesale changes in the power supply and CPU cooler.
Update: I have changed the CPU Cooler to a Zalman Noise Prevention CNPS-3000-Plus. It has helped quite a bit, the CPU fan is now silent. Unfortunately I now know how loud the Radeon GPU fan is and the chipeset fan and the power supply fan...still more work to do...