One of the toilets in the house has an intermittent issue with air
bubbles.

Occasionally, when just sitting there unused, a single bubble of air
will 'blurp' as it reaches the water surface in the bowl. It might
happen once or twice in a day, or as many as ten times (possibly
more). The toilet has zero problems as far as function go.

I think we first noticed this last year but it went away after a
while. It returned a couple months ago.

I've read that a clogged waste vent could cause this. I've looked in
them (from the roof) an don't see any blockage. The other toilets in
the house don't have this problem either.

ideas?