I just had this happen today, narrowed it down to my cellphone, not by the coil but the control box. You can replicate it by putting your phone next to the control box.
Any word on free wifi inteference at beaches? Have this one holiday beach that always gives problems, no matter what you take in the water, excal, DF, Nox.....
Has me puzzeled
I have noticed while detecting sites with short stocks of brush or stiff grass, it will cause the vdi to jump to “good” numbers when the coil bumps into this type of debris.