Back-UPS 700 ES (BE700-UK) buzzing sound - failure?

[candtalan_apc]

I have a couple of these units, about 5 years old. One appears to have failed, and makes a continuous buzzing sound. It does this when connected to the power source both with - and  without - the battery connected. The working unit makes an alarm tone - a high pitch whine - when connected to power source and with its battery disconnected. The battery in the fail unit measures 13.3 volts on the bench so it seems ok, and anyway, the buzzing is independent of the battery being in circuit. The buzzing sound could be a very rapid series of clicks, although it is a bit hard to tell.

Any comments would be appreciated here.

tia