After just over a year without issue, I noticed my S33 has started making some high pitch noise after a reboot for the past few weeks. I checked that it wasn't overheating, and I was still getting nominal performance, and the noise would go away shortly after it connected.
However, after noticing much slower data rates today than normal (less than 1/10 my typical rate), I did a reboot of the modem tonight.
Now, it won't connect at all to xfinity. It will flash a single green led then go solid amber for about 10 seconds and repeat with a lot of high pitch noise. Verified coax and ethernet cables are good.
Prior to reboot checked through xfinity that it was connected and no outages. Verified after attempted reboot via removing power for 30 seconds to the modem that xfinity isn't seeing the modem and no outages in my area. Have now re-attempted to reboot multiple times with the same result.
I finally decided to pull out my old SB6190, figuring I would have to make do with the lower bandwidth until I could file a warrenty claim. But before hooking it up and re-activating it, I decided to try the SB6190 power adapter on the S33 after noticing the 6190 adapter has a ferrite bead on it to reduce noise that the s33 adaptor does not have. Even though the 6190 adapter is 1A output instead of the 1.5A on the S33 adapter, it worked great! No more noise out of the modem and it connected at normal speeds. 
I still need to see about getting a new S33 power adapter under warrenty, but the power adaptor was clearly the key to the problem. Hope this helps someone as I've seen other posts with similar noise issues.
