Inner 1/3 of All Records Deep Obtrusive Rumble

the only thing different about the inner third of the record is cartridge alignment accuracy becomes more critical, and possibly your tone arm, or arm bearing, has some mechanical interference or roughness over that part of the arc of travel.

your cartridge could be loose and the inner part of the arc could mechanically change the level of issue to create a resonance.

tt speed issues should not be arc dependent. rumble from the motor or bearing should be the same on the outer part of the arc the same as the inner part. is the arm and bearing smooth and free over the whole arc?

check alignment, azimuth and zenith at your inner null point. make sure your spindle to pivot distance is correct and you are using the correct template.

Definitely check the cartridge alignment and overhang as previously mentioned. The KAB headshell holders have a built in overhang gauge. Sure it’s by eyeball but it works quite well. If memory serves the overhang should be 15mm. Also make sure the headshell wiring is correct on both ends. Mistakes happen….

http://kabusa.com/frameset.htm?/m1200.htm
 

Lookup Technics overhang gauge. 

The cause is not the motor unit of the Technics and the turntable is excellent value for money so not worth changing unless you are upgrading to something better - which will cost.

One thing to check is to see whether you get noise when you move the tonearm across the platter manually - with the amplifier set at your normal listening volume. I have come across situations where you can get grounding issues as the tonearm wiring twists as the arm moves across the arc of it's travel.

It's a rare enough occurrence but worth checking, along with the other checks suggested above.

It’s not inconceivable that your 1200G is defective. Have you got a stethoscope? If so, place it against the plinth and listen for a noise above background. Or borrow another 1200G if your dealer is local.