Sandman is on the money. You might also try increasing discrimination,
or changing loops. Sometimes larger loops, have a bigger problem, as
they pick up more. If your brand can use a coaxial design loop-that
is another option. They are shielded from the top and unaffected by
60 cycle interference. That design is full scan across the loop, and
while not as deep as other designs, will allow you to hunt at high
sensitivity levels.