My plumbing seems to be working o.k., but I get this annoying gurgle or glug, glug, glug in the bath tub drain each time the faucet runs in the bathroom sink and visa versa.

The toilet flushes o.k., but again I get a glug, glug, glug in the bath tub and bathroom sink drains. What is the problem? I have plunged the drains and even put drain cleaner down the pipes and nothing!!! I have had a plumber out and he seems to be confused as to what is causing this problem. I have had this problem for about a year now. We have lived in this house for over 20 years and never had a problem like this before. The house is approx 100 years old. I had the plumbing done shortly after we moved in.