Recently our downstair bathub has started burping while it drains. Seems to drain OK, but the burp, burp, burp while it's draining has me a bit suspicious. We are on sewer system, there is no bathroom above this one (although I'm not positive how the venting works). I've snaked this a little bit, but no effect, and it does seem to drain OK. I'm guessing my next stop is the roof vents for an inspection, at least.
Should I be concerned about this?
Thanks!