If emi is present you get noise; on some units I've gotten noise as the batteries get very low, and then there is inherent circuit noise. Setting the sensitivity too hot can cause noise too.
(hopefully the audio signal to noise ratio is very high) But for those causes ground balance has no effect. Some prefer to adjust the ground balance to get a weak but audible response when lowering the coil. This is adjusting for positive response-and makes pinpoint a little stronger. If the G.B. setting is too low, the response of minerals will be negative.When the coil is lowered to the ground in Pin Point or All Metal Mode, the machine will be silent. Realize that at best the ground mineral is pulling the detector silent (and the bigger the coil the more the effect) while at the same time a often small/deep conductor is attempting to put out a positive signal. With a threshold disc unit you can hear if the balance is not positive enough because it nulls; in a silent search discriminator you never hear this, but its happening.
Its a very fine balancing act, so you might need to experiment some. I always set it to hear just the slightest sound. Check your user's guide; FRL does a good job of explaining things.
(Remember one thing on the F-75 Stat mode, there is no autotune, and it drifts, so lifting and hitting retune is necessary on a regular basis.)