Light switch question for an electrician?

[fluked]

I have double 2-way switches controlling the lights on my hall and landing, so either light can be switched on/off at the top and bottom of the stairs. Recently, the upstairs light stopped working. When we tried changing the bayonet bulb, it flashed on momentarily as we started fitting it, then went dead.
The downstairs light is working normally. However, when the light is on and I test its switch with a VoltAlert 1AC-E II device, it gives intermittent beeps about 1 second apart, rather than the continual beep I get for other working light switches. The VoltAlert instructions don't explain what this means. It only happens for one of the two downstairs light switches - the other doesn't beep at all, even when switched on.
Neither of switches for the dead upstairs light make the VoltAlert beep, no matter which way I switch them.
Can anything be concluded from these observations? Is there anything I should try before phoning an electrician?

I don't know what a volt alert is but either way, I would expect it's a loose connection in one of the switches. are any of the switches physically loose, as that can cause loose connections.
To resolve, switch off all electrics at the main isolator, unscrew both face plates (and the terminal cover on the light fitting) and ensure all the terminals are secure. Then screw everything back and then power on again.

The Fluke VoltAlert 1AC II device is an AC voltage detector, but the instructions say there will be a continuous beep if voltage is present, not an intermittent beep.