"If I knock lightly" - the best way to eliminate this feedback is don't knock at all! :-)
You are creating a condition that does not exist under normal user circumstances. Most of the acoustic problems from vibrations come from airborne vibrations which are not simulated by knocking, or airborne vibrations transmitted thru the floor which again are not simulated by knocking.
I'm not sure what type of TT you are using, but I have found the best isolation for both spring and unsuspended TT's to be a solid platform of materiel with little likelyhood of resonating in the higher frequencies, placed over soft materiel, like sorbothane. You can make a sandwich if you like. Supports with air bladders instead of sorbothane materiel can work quite well also.
You are creating a condition that does not exist under normal user circumstances. Most of the acoustic problems from vibrations come from airborne vibrations which are not simulated by knocking, or airborne vibrations transmitted thru the floor which again are not simulated by knocking.
I'm not sure what type of TT you are using, but I have found the best isolation for both spring and unsuspended TT's to be a solid platform of materiel with little likelyhood of resonating in the higher frequencies, placed over soft materiel, like sorbothane. You can make a sandwich if you like. Supports with air bladders instead of sorbothane materiel can work quite well also.