If possible, have the wall shelf attached to an outside wall or strucural inside wall as mentioned(behind the speaker plane). Try to set up the shelf so that it is not rigidly coupled to the wall, don't over tighten the screws and perhaps put rubber washers as spacers behind brackets at wall. Then decouple the table from the shelf surface by putting a seperate MDF board or buthcher block on it seperating it with cones,vibrapods,small spikes,rubber/plastic pads etc. Take the lid completely off while playing(antenna for airborn vibrations) also mentioned. Give the cartridge bolts a good tightening and make sure the tonearm bearing is not loose. If this don't help I would say you need a better table.
How to eliminate TT feedback/vibration
1st Thing I will tell you - I have an OLD TT. It's a Pioneer PL-71. ( I'm looking at a Clearaudio CMB ) My TT is placed out in front and to the left of my left speaker(per Jim Smith's book.) I made a platform of MDF attached to the wall where my TT sits. At no point does the platform contact the floor. When I play certain albums-several in fact, at mid to high db levels, I get either an acoustical feedback or vibration feedback from low hz. I have tried to isolate my TT using something like sorbuthane balls and foam. I have not tried any of the commercial cone devices advertised. (Do they really work and if so-what's the best product?-no salesmen please). I don't know if my problem lies with my TT, or my platform it's sitting on. I have a friend who actually hung his TT from the ceiling years ago, and I may have to do the same. Thought I would inquire if anyone knew the answer before I started experimenting. Will a higher end TT like the Clearaudio cure my problem? Is it the platform I made-or do I simply need some isolation cones?