Why does the battery in my Volvo S80 keep draining overnight?

We have just purchased a 2002 Volvo S80 and the battery keeps draining but we can't work out why as nothing has been left on. When we try to start it up it makes a clicking noise. We had read about problems with the alarm system and had stopped locking it with the fob and then stopped locking it on the drive but the battery is still drained by the morning. There is also a faint electrical buzzing noise in the car which seems to be permanent but we can't work out what it's from.

Asked on 10 January 2017 by Angela Coleman

Answered by Honest John
Not locking it leaves systems running such as the Bluetooth searching for a paired phone and will drain the battery (though a 2002 probably won't have Bluetooth). You should always lock it. My best guess is the interior light switch. The 'delay' switch that leaves the interior light on as you get out can remain live and while the light goes off the switch drains your battery overnight. Switch the interior light off entirely.
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