I just received my new Tiger Shark. I was setting it up and air testing. I could never get the threshold to hold a steady HUM. It just pulsated like a buzzing bee. Is this normal? All of the other machines I have (Garrett GTI 1500, Sovereign GT and Excall) have a steady Hum. I just want someone experinced with this machine to tell me what a normal threshold should sound like. Any info would be appreciated.