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.
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.