This morning the shower was full of raw sewage, the toilets would flush very slowly and there was a loud bang-bang-bang in the pipes. The pipe banging has been going on for a few months most notably at 3a.m. when the water softener is activated. The toilets have clogged several times to the point where plunging them is ineffective and I have had to use liquid plumber to get them clear. The house is about 20 years old and connects to city sewer and not a septic tank.

Ok, I 'm resigned to getting a plumber out here but what is happening and what can I expect?

Hi,
I'm sure a plumber will be along to give you an idea of possibly the cost. You could also call a reputable plumber and get an idea of what it will cost to "snake" out your sewer line to where it hooks up with the main line.
My own home, after living here approx. 20 yrs, was showing the same symptoms. I was lucky, because I used a snake, from the top of the Septic Tank (I know you don't have one), and found a solid build-up in the sewage line, about 5 feet from the Septic Tank. This was stopping up everything. The clog was caused by possibly grease, and also soap deposit buildup over time. The "clog" was about 2 feet long! Needless to say, we don't pour any type of grease down the kitchen sink anymore!
I am almost sure your sewer line, from your house, to the main line, is clogged up. If you know approx. how long your sewer line is to the main line, you could buy a snake, maybe 25 feet, or might take longer. Sometimes, if you can get under your house, or maybe if you have a basement, there will be a "screw-on" plug, at a 45 degree angle, to the sewer line for snaking purposes. You could un-screw the plug, and snake it out yourself. But, if this isn't possible, or too much trouble without proper wrenches, I would call a local plumber and ask the cost. I do wish you the very best.

fredg - thanks for the advice - I just got educated even more but had to pay for the next one...

Well, the plumber just left and with him $375. He checked all the sewer lines with a camera both from the house cleanout all the way out to the street and also within the house. He couldn't find any blockages although he did comment on several possible causes.

1) the backflow valve at the cleanout might have been sticking possibly from age - he lubricated this valve with WD40
2) the existing toilets are Kohler lowflush (1.6l) style and may not have the flushing power required to open the sticking backflow valve - he suggested replacing them with Toto japanese flushing toilets @ $900 each
3) large quantities of toilet paper used with each flush - cut down on the quantity used each time