Boris, try this...
With the mode switch in all metal-motion, turn the machine on. Adjust threshold to low hum. Make sure the machine is clear of anything that is metalish, turn the sensitivity up until you get a chatter, then back off until quiet... Then, turn the GB nob two or three full turns counter clock wise, then turn it clockwise one full turn plus 1/4.
Then put her into disc mode. Turn the disc all the way down and then press the red button all the way in until you hear the threshold reset, then let go.
Now try your air-test.
I had the same experience and when I read the Manuel for start up, realized I was not "centering" the GB, which apparently you need to do for even an air test, then you want to hit the retune just to be sure. It made a difference for me. With the 8" doughnut coil, I can get a silver dime at 7 inches, breaks up at 8. A quarter is about an inch and a half better.
Also, even if the machine is not sounding off on EMI, you will probably get better results outside the house, I know I did.