"Should be rock steady at half sens. setting." That is why I mentioned the sensitivity setting possibly being set too high. If the sensitivity setting is too high the unit can be setting on the ground and will give you random tones due to electrical interference in the area.
When describing a problem it is helpful to mention what program setting, sensitivity setting, notching/discrimination, ground balance (especially ground balance), etc. I have had settings conspire to give me a tone at each end of the swing since I was swinging up away from the ground at each end of the swing.
My usual newbie mistake was cranking the sensitivity up too high. Yes it will give you more depth but it is like shining an extremely bright, extremely large light into fog. The back spill of light will make it tough to see small items but you might be able to see that white barn reflected back when it is further away. I was missing little things that were found by my buddy checking out where I'd been...
Sometimes we adjust the settings trying to use the machine without having a full understanding of what we have done. In the beginning of using my unit my best friend was the "Big Reset". You just hold down the power button until you get a double beep and let go. This will put you in the factory default settings for everything.