On Sep 1, 1:31*pm, ben wrote:
right, i've noticed no particular smell from these bubbles.

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.