When my thermostat calls for heat it turns on my circulator pump (hot water system 3 zone) the middle floor zone when it enters the first section of the convector pings very bad, this goes on for a few seconds.

As it travels through the system it comes to the second set of convectors on the same loop in another room and these convectors also ping for a few seconds but it stops at this point and none of the other rooms (3) ping...

I tried lowering the head pressure (3 floors, separate zones) tried throttling the valves (supply and return) and they still ping... ANY SUGGESTIONS

Make sure all the air is eliminated from the system.