"The beep" is right: hold the probe motion-less and still on the ground. Then gently wiggle the chord, at various spots along its length. Does it chatter with chord wiggle? If so, you might have a frayed chord. Another thing to try: while holding the probe and chord motion-less and still, tap on the junction box where the toggle switch is. Does that give falsing sound? If so, you might have some weak solder spots in that box.. Lastly, try this: holding everything motion-less and still, tap on various spots of the probe itself. Does that set it off? If so, you might have an internal problem in the probe itself. But you have to hold it absolutely motion-less for this test (even to the extent of duct taping it to a concrete surface), lest you merely be getting falses from subtle movement of the chord.
If either of these 3 things causes the flutter (in the absence of metal targets), then you might have a malufunctioning probe. Because it seems to me, that if your detector got no falsing at whatever settings you were at (sens. level, etc...), then neither should the probe.