Drain gurgling? I live in a fruit farm house, maybe 60-70 years antediluvian....

I live in a fruit farm house, maybe 60-70 years antediluvian. Every time we flush the toilet in the upstairs bathroom the drain within the bath tub gurgles. Why?
Very small white crawling parasite? Bug? Insect?? HELP!?
Answers:    check your debris vent pipe on the roof. Drop a heavy bolt or nut down it TIED TO A STRONG NYLON CORD and see if within is any blockage in the pipe IF NO BLOCKAGE SHOWS UP CONTACT A PLUMBER
Viking Hot Tubs/Spa's?
the drains (waste) are adjectives connected so you may be hearing something common. if your worried, call roto-rooter.
Whats your electric Bill?