what does "grunt cont." mean? Does "cont." mean "continuously? But anyhow, trying to describe a sound in print, is next to impossible. It'd be like saying "describe the note of C-minor in print". It can't be done. It has to be heard. So if you know of a good explorer user in your area (not just a sandbox hunter, but someone who routinely comes in with the old coins), it's a good idea to hook up with them, go out, flag signals, and cross-compare. Watch how he swings, as in speed, criss-crossing, hear what he's trying to isolate, etc....
If you were in smart screen default conductive mode, then you shouldn't have been hearing any iron. I say that because "grunt" describes the sound one hears on the explorer, if you're in ferrous mode, with the screen wide open (nothing blacked out). That is a some-times preferred mode for persons who prefer to hear when they're going over iron (but simply elect to pass up the "grunts"). It gives an idea of the amount of targets. But the mode you're in (conductive with the default smart-screen) iron should be a null (threshold momentarily goes away).
or perhaps you meant "grunt" as in a very low tone? Try this: take some small piece of foil, like a bit of a gum wrapper. Wave it in an air test in front of your coil. Is that the tone you're hearing in the field? If so, perhaps you're just in an area with lots of small foil targets?