Minusk platform

The MinusK should isolate you from everything over 2 hz. You are shorting somewhere. What turntable are you using? Check to make sure the platform is floating correctly. You should be able to tap all around the MinusK and not hear anything through the system. If you tap the top of the MinusK, the surface your turntable sits on you will hear it to one degree or another depending on the construction of your turntable. This is normal. 

I use a Vibraplane which is similar to a Minus K.  I agree with Mijostyn and I wouldn't worry too much about this if your Minus K is setup correctly. However, like Whart, I also use an HRS (with their non-spring loaded feet) or Symposium Svelte platforms.  This further improves the isolation in my experience and would reduce transmission of vibrations to the turntable.  These platforms are also supposed to help "drain" the internal vibrations from the turntable. All I know is that I like the presentation a little better with the HRS or Symposium shelves between the table and the Vibraplane. 

I have a 3/4 in quartz plate on top of the Minus K and have no ringing. The weight of my turntable/tonearms, and the quartz plate is very close to the 100 lb rating of the Minus K, which is optimal.  

Yep, it works best when it is at or close to the top of the load limit. Mine is 250 lbs and I'm pretty close. 

Thanks for all correspondence and replies.
Finally, after experimenting the issue solved. I completely removed minusk and turntable from the rack. I use artesania krion platform on top of two shelves exoteric rack. When tapping on the krion (with minusk and turntable removed)  the same metallic ringing sound remains and it comes from my tube phono preamp which stands on the lower position of the rack. Same exactly sound is produced when tapping directly on the phono preamp. But this should not happen. Krion turntable platform is considered to be isolated from the inner structure of artesania exoteric rack.