It's simple, if you are hearing excessive ground noise (or in doubt if you have the ferrous bin discriminated), rebalance (pumping auto "ground grab"). There is nothing magic about zero and each mode may react with the ground differently meaning the balance number can vary depending on mode so the number, in and of itself is relatively meaningless and is not necessarily correlated to the degree of soil mineralization.  Multi is just a little more forgiving on a less than optimal balance setting,