We're experiencing loud screetching noises from our plumbing system when the toilet flushes.?

We live in an 84 year old bungalow in the Chicago suburbs with radiator heat. There's an expansion tank on the water system. The sound starts about a minute after a flush and lasts for a half-minute or so. We can't pinpoint the origin- seems to be somewhere in the basement in the mess of pipes around our furnace and water heater. Any ideas??