Carlos, my old Gravermax classic has made that same noise for all years that I have had it. What makes the noise in mine is the motor that turns the rotary valve, the faster the SPM the more noticeable the hum, also if the unit is on without air pressure, the hum is a bit louder. I have checked the motor, and have cleaned the valve multiple times over the years, and the noise is still there, but does not effect the operation of the unit.
Of course you could always turn the music or t.v. up, then you won't hear it at all.
Carlos. I am guessing it was a "resonating" noise you had from it resting against something or the gravers sitting on top.
Mrthe. There is no "shipping screw" in the Graversmith. Only in the Mach/Max machines.