Low level hum only with lp playback

Ringing platter possibly?

Quite a few yrs ago I purchased a mat from Audioquest made of sorbojel, worked great for just what you're describing, but was toxic to vinyl, ruined many of my records.

Fast forward to just two weeks ago. Experiencing exactly what you're describing and finally tired of it I thought of a possible solution. Find one of these old Audioquest mats on eBay etc and sandwich it between my platter and mat thus protecting my vinyl.

Couldn't find one of those mats anywhere so went back on eBay and found sorbothane  ( gel )  being sold in flat squares. Purchased one 12" X  12"  X 1/4', and using my mat as a template cut out a new mat. 

Should have purchased one a bit thinner as it left my spindle a bit shorter than I liked making placing the record weight a bit more difficult.

The mat along with a few other changes have virtually eliminated any and all vibrational hum.

Sounds like you’ve isolated your problem, but my recommendation for the sorbo under mat still stands. I find it dramatically deepens the sound field along with extending it vertically. For instance, now when listening to Moby Dick live on The Song Remains The Same the perspective is standing in the field center stage looking up at the band. With my 8’ ceilings the image is a quarter hemisphere extending past 90 degrees to either side arcing overhead with back hall sounds on some recordings appearing behind the seating position. Each instrument is easily defined in space to the extent you can tell when the lead guitarist for example turns to his right or left.

I’ve made other modifications to dampen the big Kenny, will share them in another thread if anyones interested, but I have to say the synergy of the Grace arm with the Grace cartridge showed it to be one of my most fortuitous purchases to date.

Martin