[super spook]

My home was built somewhere around late 70's to early 80's. My problem just started all at once & has been doing it for the past 3 days. I thought there was a blockage somewhere in main sewer line in basement. So I ran motorized snake through & nothing.
I proceeded to run water down 3" vent stack on roof & also thru 1 1/2" vent above kitchen sink. These are the only 2 vents for everything in home. I have 1 main bath & 1 smaller with sink, toilet & small shower. The kitchen vent is approximately 25ft from 3" main vent stack. I've read several different forms & alot of them are saying I need a vent stack per toilet. Everything from sinks to tub & toilet in main bath all run into drain line & use the 3" & possibly kitchen vent also. Same goes for smaller bath goes straight to main line & no other vents are on any of these fixtures.
Like I said at the beginning this just started all at once & what gets me is this setup has worked fine for all these years. So what has changed? I've snaked all drains inside home & where it goes to city's main sewer line & no blockage anywhere. I've run out of options except call a plumber in. I'm great at doing all my own repairs. I've just gotten to the point I don't know what else to do but to install more vents. Please if anyone has any input on this I would be greatly appreciative to any & all responses. Thanks again.
I hope to get this resolved plus what done so far has helped somewhat. Just need a little more guidance.
Thanks,
Jim W. in TN