I now have a replacement for my dead Comcast DVR, and I've noticed two interesting things:
1. The hard drive doesn't chatter loudly like the previous Comcast DVR [newcomcastdvr, comcastdvrcodes] . The drive does have a slightly noticeable hum (a bit louder than the drive in my PC), but it is far less annoying than the incessant chatter.
2. The sound quality is better. I can't explain this, especially since I used the coax SPDIF connection on both DVRs. My only guess is that the audio settings on the old DVR were incorrect. For the audio settings on the new DVR's Audio Menu, I use Optimal Stereo, Advanced, No Compression, Stereo Output ( Matrix Surround).