I have a bathtub in an upstairs master bathroom. I recently noticed (couple weeks ago) that when I go to drain the tub it starts draining like normal but then after about 5 seconds it slows down dramatically. While it's draining slowly there will be an occasional sound like a big gulp of air was taken. What could cause this and how could I try fixing it?
The plumbing is about 20 years old, made from PVC. It's connected to a septic. There are two sinks nearby and a shower as well. I'm not positive if the shower shares the same drain pipes but I think they do. The only other issue I've had is that recently my kitchen island sink was burping when the dishwasher was draining. It has since stopped doing that though.
I should also note that directly beneath the tub is the laundry room.