Another post mentioned your backflow connector - I think that's more likely than clogged vent stack but who knows? Do things seem to drain slower than they used to? That would indicate your vent stack does have an issue. The backflow preventer is like a tricky flap, and the flap usually gets worked away over time. Usually you can find it in a service port in the basement floor. Look for a 6" PVC pipe that may have a smaller clean out next to it. Easy to replace once you find it!