Well, I won't go into details, but apparently when your tub starts singing, you should worry.
We checked the internet for possible reasons. There were two possible sources for our problem: blocked vent or blocked sewer line.
The blocked vent seemed simpler to fix, so to the roof we went. With a sewer snake, we checked the vent for blockage. As I was about to feed the snake into the vent, I told SSB that he should be prepared to move out of my way if a squirrel came shooting out. No squirrel, and although we did find a little compost in the vent from old leaves, not much else.
To the basement. Our access to the sewer line was easy enough to find. It is was actually in our basement next to the furnace and water heater. We did find some blockage in the line, as the snake hit a dead end. We guessed that the blockage most likely was from tree roots as there is a hug white pine right over our line.
We went to the hardware store and got some root killer. We had done some research and read that since we could have an old cast iron sewer line, we would want to stay away from the copper sulfate killers. The copper sulfate can corrode pipes and really do damage on older pipes. So we got a non-copper sulfate foaming killer.
It said that you could flush it down the toilet, but I wasn't willing to take the chance of this "foaming" chemical frothing out of my toilet all over the bathroom. We just dumped it into the sewer access, screwed the cover back on and then flushed the toilet a couple of times. It took two bottles of the stuff put in the line a week apart, and we were actually thinking that we were going to have to call the sewer guys, but SSB was in the basement checking the laundry tub while the washer drained (we had to make sure that we weren't getting overflow when ever we drained the tub, sink or washer; and we could only do a 1/2 load of laundry every 8 hours!) and it started to fill as usual, then he heard a "whoosh" and we have been clear ever since.
I never thought I'd be so happy to actually do laundry!
What a story -wh0000000ssszzzzsetth -what a relief !! drop a line to the chemical manufacturer, they might want you to do a paid commercial !!
Best wishes
Lindsay, not a bad idea. Maybe they will give us some coupons. We have to put the chemical in our line at least once a year.
We thought being without water while we were tiling the bathroom was bad -- this was far worse. I am so happy to have that over with!