DIAGNOSIS Yeah, this one's common and startling. That's the hub's internal alarm going off. It has two triggers: a low battery warning for its backup battery, or it thinks it's being tampered with (moved).
FIX The most likely culprit is the rechargeable backup battery inside the hub. It's supposed to charge from the wall power, but over time it can fail. Unplug the hub, then unscrew the back plate. You'll see a small, rectangular battery pack plugged into the main board. Unplug it, wait 30 seconds, and plug it back in. Reassemble and plug the hub in. The beeping should stop as it recharges.
IF THAT DIDN'T WORK If it keeps beeping after a day, the internal battery is probably dead and needs replacement. Chamberlain doesn't sell it as a user-serviceable part, but you can find the specific 3.7V lithium pack online. Replacing it will stop the alarm.
HEADS UP If the hub is physically loose on the wall or ceiling, vibration from the door opening can trigger the "tamper" alarm. Make sure it's screwed in securely.