Hi, this is and idler wheel drive turntable that means a small rubber idler wheel is used to spin the platter. This idler wheel touches both the motor and a rim on the underside of the platter. What has most likely happened is that the rubber idler wheel has turned hard as plastic from age. It should be soft and pliable not hard. Good thing is that the idler wheel can be rebuilt or replaced. Once this is done and the turntable serviced correctly it should run quietly. From what I have been told who ever made these idler wheels for zenith did not vulcanise the rubber and it just turns to plastic over the yrs.
You will need to remove the platter and check the idler wheel. If it's hard then remove it and send it in to be rebuilt or replaced. One of the best places I have found for this is http://www.thevoiceofmusic.com
. Over the yrs I have sent Gary Stork who runs this site quite a few idler wheels to be rebuilt. He has never let me down and his prices are fair. I hope this helps.
Idlers, belts, motor mounts, wire sheathing, etc. naturally deteriorate
over the years, just like in cars.
Chemical changes happen, outgassing, acids in the air have effect, it's natural.