This is a weird one, at least to me.

Night before last, we had a clog. Had the plumbers out yesterday and it took a while but they cleared it. However, early this morning I was sitting on the toilet and the heater came on (electric furnace), and when it did the sink next to me made a gurgling sound. The heater went off and it stopped. I was able to replicate it by getting the heat to come on and go off again.

Thing is, this bathroom is in an addition to the house, and while it does share a common drain pipe to the city sewer, the sewer line from the addition doesn't join that line until it gets around to the front yard. It goes all the way around the house. The plumbers didn't find a problem with that "system", and it was working fine when the kitchen sink was clogged. And it seems to work fine now, honestly. Just noisy when the heat comes on.

So I go outside and look around. The cleanout they used at the back of the house by the kitchen was closed up. The one for the addtion was closed up. But the other one wasn't. They forgot to put the cap on.

So I'm thinking and hoping that just some air got in the line with the cap off like that, and it will go away. But it seems strange. No other drains are making noises. It does seem like the kitchen sink is draining slowly again, but that could be my imagination.

Is my theory plausible? Will this sound go away? Or do I need to call the plumbers back?

Thanks.

Edit: the sound grew fainter and fainter and is now gone.