On various occasions over the past 30 years, I have had partial
blockages in my sewer pipe which have caused the basement toilet to
gurgle and bubble when the 2nd floor toilet is flushed. I snaked it
out last year and have been flushing rock salt through the basement
toilet to keep the roots at bay.

We went away for a week's vacation and when I came back I noticed that
the basement toilet was bubbling and gurgling when the 2nd floor
toilet was flushed. It had not done this before we left so I was kind
of surprised.

I had resigned myself to the fact that I was going have to rent a
power snake this weekend when the problem went away. It has not
gurgled or bubbled for 2 days now.

I can only assume that things dried up while we were on vacation and
the waste was not flowing as well as it should have. 3 days of use by
5 residents and things appear to be back to normal.

Interesting.