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?