We've had a small issue possibly related to venting in our house. It's a bi-Level with a half bath downstairs. In that half bath, the sink has always "gurgels" a little after use. However, recently we've noticed that when we dump a large amount of water down the sink upstairs in our kitchen, our toilet in that half bath bubbles. Yesterday we dumped soapy water down the drain in the sink upstairs and suds actually bubbled up in the toilet downstairs. Could this be a venting issue? As far as I know, the vent stack hasnt been cleaned or flushed in a long time.