We have a Music Man Sterling 4-string bass that has been an outstanding instrument here in our church for a number of years. However, in the past few weeks it has developed an annoying problem. At certain times it is much louder on specific notes-particularly the C note on the A string and C note on the E string. We have changed the strings and the 9V battery to no avail. Any suggestions to this non-bass playing guy would be most appreciated!