How can fix banging in hot water boiler system?

I have older Burnham Anerican bioler heating system
(31 years) the fill valve quit working and relief valve
starting to leak and the expanison tank had rust spots
so I had every thing reworked including new plumbing

now when there is more then one zone working and one zone shuts off we get very loud banging if there is only 1 zone on and it shuts off and there is no banging
I think it is the expanison tank but my repair man says
it is ok . do make differnt sizes ex tanks

Sherrill West
January 2011