Google Image for “plumbing vent” and you’ll see lots of diagrams of how plumbing stacks should be laid out…
Is the light fixture below the bathroom? Or is this a 1-storey cottage?
Toilet flushes itself is something sucking the water down the pipe. Usually, this is due to a slug or rush of water down the drain and no ventilation, sucking everything in its path along with it.
As others said - bubbles? sounds like very poor drainage.
A possibility is that the is a stretch of pipe that is horizontal but actually slopes the wrong way a bit, so where it should drain and vent, it is creating a plug like a U-shaped trap does, instead of allowing venting.
If it is 1 storey, then maybe what is happening is back pressure (air buildup in septic tank?) forces drain water out the vent stack, and previous owner left the vent stack to vent in the attic rather than through the roof.
I sure hope it’s clean water, not sewage!
Best thing you could do is try to make a plan of exactly how the drain system is laid out. If the light fixture is from the attic, crawl around the attic and figure out what pipes are there.
You should have a vent going up from every collection of drains - one or more for the bathroom, one for the kitchen sink (unless it backs onto the bathroom and shares it) one for the laundry tub and washer drain, etc. These should obviously be poking through the roof into the sunshine.
I sure hope the guy did not leave a vent stack ready to continue up, a foot or so above the floor line, then walled it in forgetting to extend it to the roof. Does the house sometimes smell like swamp gas?