Help -gurgling noise, slight backflow from first floor tub.

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?
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Tony Wesztergom wrote:

Since it sounds like flushing a toilet doesn't affect it, the problem is not in the Main line to the street or septic (or in the main stack).
Plunging the tub may not be effective enough. If the clog is past where the shower joins the line for the tub, plunging will likely just send air up the vent.
Someone will have to make a guess about the piping layout and use a small snake to clear it. I'd hesitate to diagnose it any further than that over the 'Net.
Jim
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