Thanks for the comments, everyone.

No, the mount is not covered by warranty. I bought it used, and was surprised to discover I've had it since September 2007--doesn't seem like it's been that long. Usually makes it out of the house once or twice a month, so it's gotten some use.

I'm going to kick myself if it's just a power issue. It was the second night of observing, but I don't think the first night's observing was long enough to wear down my car-jump battery. It's a big one--17 amp/hours. Also, when I plugged the battery for recharge this afternoon, the little push-button indicator said it was putting out 12.5 volts, which means the battery was down a bit but not that much.

The noise hit me on both DEC (during a slew) and RA (during just tracking, after I stopped trying to use goto on account of the noise and the stop in DEC slewing). Because it happened during tracking, I'm less inclined to think it was a weak power supply, but we'll see.

Boxed it up and sent it to Orion this afternoon. When I hear back from them, I'll update you all.