Car won't start - loud buzzing moise

Yesterday I moved my D2 about 100 yards then stopped & switched the engine off & removed the ignition key. There was a continuous loud buzzing coming from the direction of the gearbox which only stopped when the battery earth lead was removed. It recurred as soon as the bty lead was replaced. When trying to start the car,. the engine beqan to turn momentarily then stopped & the buzzing recurred at a rather lower frequency. On closer evaluation the noise is comong from beneath the passenger seat (N/s it's RHD). A local L/r expert suggests a compressor problem - the car has airbags.I suspect a solenoid failure but in nearly 60 years' working with infernal combustion engines have never experienced anything like this! Advice please.

Remove relay R5 from engine bay and see if the noise stops or not, if it stops put R3(headlamp wash) instead, if it's OK this way means that R5 is sticking so you need a new one , if with R3 is the same we'll speak then but if it's on good level you can unplug all the connectors in the compressor housing and drive it with relay removed, if there's no leak it will stay on level for many days just that the SLS warning will be on which if it's facelift is annoying as it comes with continuous bongs

Thanks - a local expert advised removing R5 as well: the buzzing stopped; the engine will now start. As I have limited tools & the weather is foul I'm taking the car to said expert to fix.
You advised a few weeks ago about a voltage drop problem which I then fixed; however it recurred recently. Recleaning the plug & socket didn't cure it this time so I've been advised to replace the fuseboard.