I am afraid of our plumbing

OK, you've read the previous post about respecting drains. I have the utmost respect for drains. I get very anxious about drainage problems.

And we have another one. This evening, upon flushing the upstairs toilet, there was a terrible gurgling. But it didn't come from the toilet. It came from the drain that is inside the wall that was once used to drain a washing machine that is no longer there. It's actually a fair distance away, across the bathroom, and behind a cabinet door. Several more flushes yielded the exact same result -- a successful flush, but a bellowing burbling glug glug glug from the un-used drain.

My unprofessional google-based opinion is that it's a problem with an air vent. Long-time readers will remember that when we moved in, the plumbing vent situation in the attic was of concern:
http://83durant.blogspot.com/2007/01/attic-mysteries.html

We solved the open-vent-in-the-attic problem by installing an air admittance valve:
http://83durant.blogspot.com/2007/03/preventing-attic-moisture.html

It's possible that the air admittance valve is now malfunctioning. Or maybe it's something else entirely. That's why I blog, in an attempt to more productively direct my obsessive fears about our house. To the plumbers among you in our readership: any suggestions?