There is always MDC, and I'm well aware of what that sounds like, but there was another beep of some kind on the repeater also...I noticed today on the way in that it was gone, and I'm listening to them on the desk scanner and it's not there either. Wierd.
Some repeaters are set up to add a courtesy beep when they are on battery backup power. It's an indicator to users to keep transmissions short (not to drain the batteries) and get someone enroute to start a generator, if needed. Not sure if that's the case there, but I've heard it in other areas.