A couple of weeks ago I sent to a site to investigate a faulty emergency light. It was 'buzzing'. Turns out it was a rather clever self-testing unit (this style), and it was reporting a problem.
I checked the batteries, fine. Checked the bulbs, fine. It must be the electronics so I contacted our office and ordered a replacement board.
The other day I went to install the replacement (they sent the whole unit), which went fine. I took the faulty one away with me but rather than throw it out I thought I'd take it home and play with it. At the very least I could use the lights on my car.
I believed that the fault was probably in the self-test module rather than the charger/switchover unit, and as I was looking for an emergency light for my cellar then I might just be able to use it. I didn't really need the self-test function anyway so I'd just remove that and rewire it as a normal unit.
Got it home and poked around with it, only to find that the fault was just one of the batteries after all.
I'd measured them both, but only by chance checked the bulbs on the good one. The bad one had voltage but couldn't produce any current.
So I fitted new batteries and it works perfectly.
The buzzing was merely the self-test unit reporting the faulty battery, which I had failed to diagnose.