No joy - replaced the bottom right relay ok, switched on - seemed fine for 5 hours, then the buzzing came back! It's as if the relay is opening/closing quickly hence the vibration being felt... Any help / advice appreciated.

Paul

 

Then you need to diagnose the signal going to the relay.
Max.

 

It may be that that the relay is powered by unregulated DC and one, or both, of those large caps is failing (quite likely in a warm environment) leading to a large amount of ripple on the supply. The electronics may be taken from the same supply but regulated down to 5V so are unaffected. If you set your voltmeter to AC and check the supply to the relay any ripple will be apparent.

 

The relay board looks fairly simple to reverse engineer, with little effort you may be able to swap the functions of two of the relays and see if the problem transfers to another relay, the 24vdc supply to the relay board appears to be the blue and blk wires?
Max.

 

It's worth checking the relay voltage when buzzing and not buzzing.

@MaxHeadRoom
You made a boo boo.. 2VDC to power the relay..!!!!

 

@MaxHeadRoom
You made a boo boo.. 2VDC to power the relay..!!!!

Thanks
Max.

 

It's quite possibly the transistor that activates the relay's coil... or maybe the value of the resistor at that transistor's gate is a bit too high