Turntable feedback

if you want stone that has a natural damping feature get slate. costly though. 

I've been through all this over the last couple years as I put together my Garrard 301. I was originally going to go with a stone, probably slate, but its quite expensive to get and harder to work with.  I finally settled on Bamboo ply layered.  I had a slight issues with my granite slab transferring noise to my table when it was on pin points. I tried hard, I tired soft isolation schemes ( many of each type) I eventually forked over the cash for the Ios Acoustic footers and problem solved. I even took the granite slab out but the loss in detail was not worth it so I found another solution in the Iso Acoustic Orea.  my tables 100lb'ish sitting on that dam 165lb slab lol I don't have rumble issues lol. bad back defiantly.  

I've seen this work many times......Put an appropriate piece of wood between the table your turntable is on and the wall in back.  Tightly wedge it in place. Let the results be known

Schmitty1 it sounds like the cartridge you are using may be too stiff for your tone arm. You might try putting a more compliant cartridge in there and see if your problem goes away. I have seen instances where bass on the recording was not enough to set off feedback but get enough bass going in the room and away you go. You might also have inadvertently located your turntable in a node so moving it just a few feet might make it less prone but still given enough volume it will feed back. The primary problem is the resonance frequency of the tonearm cartridge combination which is being excited. It should be down between 8 and 12 Hz. A good test record will give you that information.You obviously have a very sturdy platform. I doubt it is the problem.