New Stand config brings about new TT problems

I recently bought a new plasma TV to help make my shared Living Room/listening room look more presentable. Prior to this we had a 55" big screen TV with the 5 shelf stand to the right side of the TV
Now the stand is lower and the big box TV is gone and the plasma hung on the wall. However I am getting a lot of noise when tapping on the TT plinth. This was not the case prior to this new config. I am assuming that the problem is sonic feedback and that the old big screen TV was helping to shield the TT from most of the feedback problem. However, if correct I'm not sure what to do about it short of a dust cover. And maybe that is the answer. The new config picys are shown in my system here, while the old config is shown
>here

FWIW, I am using a sand box now as before. In an effort to figure out this problem, I've removed the 3" maple shelf shown in the pictures and gone back to the 1" maple with separate motor platform. I've also changed the mini TNT feet back to the stock feet. There seems to have been little or no change
So therefore I'm still assuming sonic feedback. Suggestions, insights, etc will be appreciated.