Taf:

Cupping is hard to describe, but here goes...... run your hand over the
tread on the inner and outer edges.  It should feel relatively smooth with
the only roughness being caused by the tread pattern.  Now reverse the
direction that you rotate your hand. Is it still smooth, or can you feel
some rough edges?  If you can, it's probably cupping.  In severe cases you
can actually see the cupped area on the tread.  This can cause the wow-wow
noise you describe.  Most often cupping is caused by infrequent tire
rotation.  You should rotate your tires every 6K miles for maximum wear and
to avoid cupping.  Check tire pressures every 30 days.  When rotating use a
cross to the rear pattern.

The best way to tell is to rotate em back to where they were and see if the
noise goes away.

As a "fix", you can leave 'em where they are at and eventually unless they
are terribly cupped, they will wear back to a somewhat normal condition.

Hope this info helps

Mike