Hi! I just bought my first Minelab detector, an used Explorer XS. It looks in good cosmetic condition and seems to work good except one thing that maybe is normal or maybe not. The threshold sound is fine when I turn the detector on, but after some time detecting it becomes like a broken sound, not an stable "hum". All the other sounds (detection sounds are ok.) Any ideas, it is normal, is there a setting that I need to adjust to avoid this? Or it is faulty? I am a complete newbie with this machine, not to metal detection. I'm unly using quickstart by the moment. Thanks!