Our tub began making gurgling noises when the toilet flushes as well as our sink becoming slow to drain. I then Checked the vent stack and sent water down to clear any obstruction. Still gurgling. I snaked the toilet with a 3-4' snake as well as taking off the sink trap and snaking all 3-4' into the wall. Still gurgling and sink is still slow. I plunged the toilet, tub, and sink. Still Gurgling. Toilet fine and tub drains fine, sink is slow still and tub still gurgles at toilet flush. Also, the downstairs sink and toilet flush/drain fine with no noise. Help please !