Cleaning the air valve on the vented loop didn't work, so I think it's time to replace it....the duckbill valves in 'em do have a finite lifespan. If you don't know your loop's brand name, I should be able to ID it if you'll post a photo. No need to remove anything, I just need to see what the "cap" looks like.
And btw, I'm 90% certain that the toilet isn't the culprit...that it has to be in the plumbing and the air valve in vented loop is the only thing IN the intake plumbing.
--Peggie