occasional air bubble in toilet

[ben]

right, i've noticed no particular smell from these bubbles.

[jamesgangnc]

I don't think you can use smell to confirm anything. If the plumbing is vented properly and in working order any atmospheric pressure in the drain pipe should be vented out the vent at the top. It should not be forcing air back out of the toilet. It's an old house so either improperly installed plumbing or a clog could be the explanation.