Re: bubbling in toilet when bath empties
Clogged vent --- maybe.
Classic vent problem would be whenever the toilet is flushed you hear gurggling from the tub / sink drain.
I would also expect to hear gurggling from the sink drain when the bath is emptying since it would be the first thing in line to vent , rather than the toilet.
Question -- are you also experiencing with the toilet flushing?
If so , there might be a partial blockage where the bathroom fixtures meet the main vertical stack. In this situation the bubbling in the toilet would be the air in the drain that's ahead of the water flow , being forced back up the toilet. If the water in the bowl is disturbed violently enough it can spill over the siphon trap and empty.
"" an ounce of perception -- a pound of obscure "