I have a toilet on the ground floor of a 3 story 100 year-old building which was converted to condos in the 80's. Recently large bubbles of air splash up into the bowl, often repetitively, sometimes for a minute or more, without my flushing the toilet or using any of the plumbing in my unit. If I flush my toilet soon after this happens, it backs up nearly to the rim, then slowly drains over the next several minutes. Flushing the toilet when these loud, large bubbles haven't occurred for awhile seems to be perfectly normal, but I'm afraid this is only going to get worse with time. No one else in the building reports any plumbing problems. Does anyone have any idea what the problem might be, and what I can do about it? A friend's husband is a plumber, and he says he's stumped, so I'm a little reluctant to call a plumber...
