Looking for some plumbing help here. Our house has 2 full baths which
share the same common line. We use the second floor tub/shower, but
not the first floor. Over the last few days we've noticed that the
first floor tub makes "gurgling" sounds and has a slight backflow into
the tub while someone is taking a shower on the 2nd floor. The
drainage in the second floor tub is ok, but there can still be some
pooling while taking a shower. I have cleaned out and built up hair,
and have also plunged the tub, which helped drainage for a few days.

Is there anything I should or can check to see where the problem is,
or should I contact a plumber?