Thanks for all the good advice - I learned from that and the suggested URLs. One thing I already knew but ****hiller reminded me of: "Whenever you have a new problem, look to the last place where work was done." Problem was air trapped in the new circuit for the hydrocoil. I powered that pump directly and let it run continuously for 2 days then there was no more air noise and no zones stopped. About 4 days ago I closed the make-up water valve and still no problems so I guess I don't have a leak either.
Bottom line: contractor didn't purge adequately after installing the circuit for the new hydrocoil. He paid dearly by coming to our house 4 times at night to re-purge other zones to get the heat going again. Why was this system so difficult to purge? Dunno. Maybe since the new circuit still isn't in service (pending a special control), it wasn't adequately purged and burped air from time to time.
For now this gremlin is gone and I hope for good.