TT Hum

au_lait, when it talks like a duck and walks like a duck it is usually a duck.
The Cartridge is not well shielded and there are electronics at the right side of the table that are emitting. Make sure the cartridge body is getting grounded. You may need a cartridge with better shielding. It is not too bad and you do not hear it through your speakers so you may elect to just live with it. 

Mijo, I ask this out of curiosity, because you raise an interesting question: how many cartridge bodies are grounded or even can be grounded, unless there is continuity between the cartridge body and one of the  audio grounds available at the pins?  As you know, many cartridges are made of plastic or other non-conductive materials.  Also, many headshells are non-conductive (made of wood or carbon fiber, for example), so grounding through the headshell is often not feasible.  For that matter, the only cartridges that would be shielded would be those with conductive metal bodies that are then grounded preferably to the chassis.  I am not implying that your advice is not good.  I am just curious whether I am missing the point.  Thanks.
This is not at all to say that you are not correct in guessing that the OP's cartridge is picking up some radiation from the electronic panel under the tonearm.

@au_lait

-What is the cartridge?

-If you have a few different cartridges do you have a hum with all of them?

-Any proper platter mat on your Technics SL1200 or just a felt slipmat or stock rubber mat ?

-Hum is noticeable on normal level or on high volume only ?

-Can you hear the hum only using your headphones and its amp or you can hear it in the speakers?

P.S. First thing to check is ground wire from your Jelco tonearm to your phono stage. You can disconnect it and try again to see is the hum is gone or increased. You have to check headshell leadwires and headshell connection (lock) with the arm. DIN connection of the phono cable to the tonearm base. This is basics. Put some nice metal mat on Technics platter and try again.

@chakster Yeah I have done/checked every one of those basic tests and have narrowed it down. It's not a ground issue - my phono cable grounded to either phonostage or preamp is all good. Its a very quiet hum (not buzz) and it really only happens when the arm is resting. Not hearing through the speakers unless its really loud, BUT I am digitizing lots of vinyl and I hear it a bit through the ADC on headpones. It's not the end of the world, of course, and I will try another cart tonight. Just curious if anyone had experienced this before. Maybe I lowered my noise floor to the point that I've uncovered something that was masked before?

Cartridge: Nagaoka MP150 (Will try with Zu DL103 tonight)
Mats: Nagaoka Crystal, will try with Herbie's, TMM Monitor, Rubber/Cork mix tonight

Almost any cartridge will produce a faint noise if it is in space, not touching the LP surface, and if you then turn up the gain high enough. That is more a function of the phono stage signal to noise ratio and/or ambient electrical noise, than it is a function of the cartridge.  But on the one hand you do say it only happens when the arm is resting, which fits my theory.  Then you imply you hear the faint noise on your digitized copies of LPs, where the stylus must have been tracing a groove, which contradicts my theory.  Which is it?  The cartridge hanging in space is like an antenna which picks up stuff and then delivers it to your phono stage, which has a lot of inherent gain.