Yeah, gotta be carefull of those. I like my digging tool or screwdriver and you can hear the metal scraping on the glass, makes a pretty specific type of sound.
I bought a Fisher F22 from Amazon (I'm noob - just started) and based on my small number of outings, it does seem as though it fluctuates quite a bit and will sometimes lose a signal completely - I'm taking it to Hills End in a week or so to see what I can find - I'm hoping that with a bit more familiarity I'll have better luck reading this thing.