We have a two story house, two full baths up, laundry up, one half bath and kitchen down, on a slab.

Moved in five years ago, had the septic pumped a year and a half ago when cleaning a plugged main drain line. Said, based on their records, it wouldn't need it for another four years or so.

A couple of weeks ago, I started noticing bubbles from second upstairs toilet bowl. It bubbles whenever the washer empties or the other shower is used. Basically, on the upper floor, my guess as to the arrangement is that the first bathroom (no bubbles) feeds toward the laundry room, then on to the second bathroom where the stack drops through a wall The downstairs bath and kitchen join the plumbing under the slab, headed for the septic.

The downstairs toilet, with is near where the stack drops down, shows no bubbles, but it does seem that the level gradually drops in the bowl as other water is drained. (Yes, it drops, it never rises above normal-not a typo)

Since significant water flow from either of the first two makes the toilet (next in line) bubble, it seems that the problem is likely between that toilet and where the rest of the plumbing joins in the slab.

It never did anything like this before.

The kitchen drain has a cheater vent under the sink, the rest all vent through the roof. (Sorry, I haven't mapped out what vents where yet)

My question, is it more likely a vent plugging issue, which I would certainly want to hire out, or a plug in the drain line, which I could likely take care of.

Anything I can check to narrow down which one?

Thanks!