OK, I spent time trying to figure out what happens. I observed that when at max (thr, sens and min disc) it is very unstable making false target sounds. Hmmmm
Here is what I found inside the control unit: from coil connector there 2 pair of wires and ground. Ground is black, then red-black and green-white. One is obviously transmitter pair, another receiver. Green-white twisted just 2 times over and (here is the key) laying over another pair touching it. Here is my theory: Frequency is very low, I get it, they twisted (just 2 turns), but still 2'' wires act like antennas where transmitter inducts magnetic field to receiver messing the signal.
The fix: I just bended pairs opposite ways and made sure they stay far when closing. Now it is way more quiet when pushing to limits, and most important -- I got 8.5'' stable reading on a dime in air test. Problem solved. Hope this will help someone.