I have a toilet in my room downstairs that sewer gas comes out of inside the bowl. Actually bubbles and sewer waste fills the bowl. This doesn't happen in any other toilets in the house upstairs (we have 3 upstairs). I think it's mostly when laundry is being done upstairs. :confused: I'm pretty handy and can replace toilets and so forth. But this is a mystery. Can it be a vent clog?

Often this happens when the sewer is (partly) blocked.
The water from the laundry section can't flow towards the main sewer fast enough and backs up towards the toilet you refer to also, causing the trapped air to escape via the toilet bowl. If it gets worse the next phase is the bowl water level to raise till it overflows.

It also could be caused by a blocked air vent pipe (the one that is running to the roof).
But my money is on the first suggestion of a partly blocked sewer pipe.
That stated : I am not a plumber, just advice from my own experience in my previous house, where we had identical problems.

Success !

:)

I am going with the blocked vent. The toilet is acting as a vent when water rushes into a drain the air/gas must be displaced. If your stack is blocked the air looks for the next easiest exit. Rod from the roof with enough snake to reach the basement floor and then 15' more.

And using too much detergent or a high sudsing kind can make soap suds back up into the air vent when the washer is draining. The column of suds can be 8-10 feet high at times. Has your washer maybe eaten a few of your socks?