I had a coil that when set flat on the ground (full face of coil) it sounded off loud and kept sounding. It was a bad connection on the shield wire. If that is what your coil is doing, call the factory.
With the Tejon, when sensitivity is set high and there are interfering signals in the area, it can chatter with the coil stationary and pointed towards air and not the ground. It quiets when the coil is pointed towards the ground and the detector is reading the ground matrix. This is a chatter of short sharp little beeps, not a loud long tone like you are are six inches away from a steel car body.
Cheers,
tvr