check your stink pipe on top of your roof and might have a bird nest or something in it where your not getting air for drains to drain and when the water is ran upstairs well then it has to drain downstairs and it is gurgling for air and getting it from the commode -- either way somewhere in between up and down you have a clogged/clogging pipe -- whether air or are actual water drain out probably a old galvanized pipe
Sounds like you may have a blocked vent. Pluming systems have vents that help to vent gasses and equalize the system. You can see the vents that stick through your roof. Sometimes a vent will be blocked with bird nests or wasp nests. If you do not have a plumbers snake and know how rod out the vents, call a plumber. Clogged vents can make your house stink as the gasses escape inside instead of out.