My basement toilet gurgles when the up stairs toilet is flushed. What's the problem?

my basement toilet gurgles when the up stairs toilet is flushed. whats the problem?
Let me explain about bubbles and gurgles. "Bubbles" and "Gurgles". They both indicate that something is going on in your drainage system.
A "bubble" indicates a clogged line while a "gurgle" indicates a blocked vent. Let me explain. When you flush the discharge drains down the pipe until it hits a partial clog. Then it rebounds back sending a "bubble" of air ahead of it. When you flush a toilet or drain a bathtub a suction is created by the water rushing down the pipe. This suction is relieved by a open pipe that runs to the outside called a vent. When a vent's stopped up the suction has to relieve itself somewhere, in this case your toilet. The noise you hear is the air being pulled through the trap by suction, (gurgle).
Rule of thumb! Bubbles= drainage clog, while Gurgles = a blocked vent or poor venting.
Ifyou have a lavatory in the basement then snake from the lavatory roof vent. I'm assuming the basement toiletr's vented\ or you would have been hearing this since day one. Good luck, Tom

There should be a vent stack above those 2 baths on the roof. Shine a flashlight down from the roof. Look for an obstruction like a birds nest or ball of leaves!