How to properly isolate a turntable?

As Metralla suggests, so long as its not airborne it should be easy to isolate your table from floor borne vibrations.

If you are into home made (cheap) solutions I would take 2 types of materiel that will absorb vibrations and make a sandwich of it under your turntable. I would use something like (or actually) sorbothane between your stand surface and a sheet of something relatively hard like, well, hard wood. That should kill most incoming vibrations being transmitted thru your floors/stand. At least it won't cost much to find out if your going in the right direction before you buy anything well thought out and available commercially. If you have to start off commercially test before you buy to be sure that corrects your problem. This sandwich concept is well know and is even incorporated by some TT manufacturers into the base of the TT's which are not suspended (as yours).
Now if your problem is 'air borne' you might fix it by relocating your TT to somewhere in the room with minimal bass nodes, that is probably the best and cheapest solution. Again, try this before you start buying fixes that might or might not work.

Lastly, you might have a set up problem, that is your have a mis-match between the mass of your arm and the compliance of the cartridge, enuf so the effect trickles up into the audible frequency when there is also strong low bass energy from the recording and or system. I doubt it, but its possible.

And if that don't work, stop 'dropping' the needle! (Sorry I couldn't resist.)

do you have a suspended table? a lid for the the table?

Only way I got rid of my feedback years ago was to put the table in the next room. this was with a Ariston Rd 110 similar to linn.

I am guessing that you don't want to spend big bucks. You might try and find a used Bright Star Audio Air Mass. If you do use an air blader system, it is imperative to keep it level. There are othe types of isolation devices descussed here on Agon or other sites as well. Good Luck.

There are many approcahes depeding upon your intended cost.

Place a good quality shelf under the turntable, such as a Neuance, Mapleshade, or a Black Diamond. You can even go homebrew with a slab of MDF. Then use a sheet of heavy-duty (the pink antistatic) bubble-wrap between your rack & the isolation shelf. This is a proven approach used by some highly knowledgable members here.

Instead of the bubblewrap you can set the shelf atop a set of Vibrapods, superballs, sorbathane pucks, etc. Be prepared to experiment for your prefered sonic signature.