Hi Simon, maybe you're on to something here. Is there any chance what you are hearing is the 50Hz mains? Arms placed behind the platter are obviously nearer the motor and can be more susceptible to picking up noise but that's usually rumble. Do you have an alternative tonearm to try? Given you have swapped/serviced all the Lenco parts maybe the problem isn't with the Lenco What cartridge are you using - not a Grado by chance?Not sure I would call it rumble now, it's more like there is a frequency being picked up from the motor and amplified. It's not a hum as per a grounding loop but something much bassier??
I did wonder about the proximity of the unshielded tone arm leads from the WTA when in the rear position but moving it to the usual right hand side
position improves matters slightly but not by much.