This recently just started. When someone used the sink OR toilet in the bathroom the sink in the kitchen gurgles. The kitchen sink has 2 drains, one has a garbage disposal. The gurgling come from the one without the disposal. I ran the bathroom sink and noticed water coming up the kitchen sink drain in waves--it would come up and then a bunch of bubbles would come up, then the water would go down and then back up. It never came as far as to actually make it into the sink itself.
I've also now noticed that when the diswasher runs, (which drains into the garbage disposal) same thing happens.
I kinda wanted to get people's opinions on what the problem could be before I called a plumber. Last time I had a similar problem that wasn't actually a blockage wound up costing $10K of work when the plumber decided to snake the line anyway and discovered some construction debris in the line.