Basement toilet Gurgling
Sounds like a problem in the venting.  I don't grok drain venting well enough to know if it's the upstairs toilet or the downstairs one that's not properly vented (maybe both), but the rush of air getting pushed ahead of the flush needs to go somewhere, and that's what the vent stack is for.  If the vent is clogged or nonexistant, the air will find the outlet it can (in this case, the downstairs toilet).