Thanks for the reassurance Kesh, I'm kinda new to this, so just wanted to make sure I hadn't messed it up already!
I think I have found the cause of the motor hum!...It appears the motor support plate was rubbing against a protruding section of the plinth adjacent to the motor.
(The plinth of my deck was 'custom-made' in wood, not plywood, by the previous owner, which looks good, but is probably not the best material to use as it seemed 'resonant' from the start...but that's another issue!)
Anyway, what I was probaby hearing was the whole plinth vibrating and amplfiying the hum!...as soon as I made sure the plate was not rubbing the hum disappeared!
After removing that 'annoying' part of the plinth, I shall turn the deck up the right way, and try to access the suspension springs from the top to check that the damping foam is in place. (I take it I would have to temporarily remove the cable restraint bracket from the back to allow me to do this).
What would be the best type of foam to use for this purpose?