I have a 70 year old house (connected to city sewage) that has a toilet in the basement, along with several floor drains (one intended to be a shower pad), and 2 main stacks. Recently, I've noticed the toilet in the basement gurgle when the water is being run down the utility sink near by. It also does it when the washer (also near by) is draining. I recall this happening a year or so ago, but it cleared up on it's own. Nothing but liquid is ever flushed down this toilet, so I don't believe it is plugged. And the drains that are causing the issue go into the main stack about 16 feet away. My toilet on my main (only) floor does not do this. Additionally, I haven't noticed any draining issues with my kitchen sink, bathroom sink, or tub.
I've attached a rough drawing oh how things are laid out in my basement (not to scale by any means). I'm not entirely sure how the pipes are connected in the basement slab. I presume its a fairly straight shot from the toilet out to the sewer, with the main stacks coming in slightly perpendicular. The drain line for the sink and washer is 2" PVC. There is a studor vent right off the sink as the person who installed it was concerned about the long run to the stack/vent.
Edit: Additional info/observation. I flushed the toilet, and it did back up towards the rim, like the toilet was clogged. But again I know the toilet itself really can't be, as no solids go into this toilet. Also, when the gurgling occurs in the toilet, I never see air bubbles come up in the bowl. However, the water in the bowl does start moving, sort of like waves on a lake.
Anyone have any ideas on what could be causing this?