Pipe from gas boiler making loud noise (link to sound)

[tundea99]

Hi,

I have gas baseboard hot water heat with two taco zone valves. One zone in particular makes a loud knocking noise whenever the thermostat calls for heat. The noise is coming from the first part of the zone right where the zone valve is. I've been in the house for one full winter and I've never heard the noise until last week. I've had a guy come and drain the system as he thought there might be air in the system but that didn't solve the issue. Does anyone have a suggestion? Here's a link to the noise:

Thanks,

 

[Fitter30]

There are two or three o rings, a seat in the valve and a spring. Spring might be broke. They make rebuild kits but a whole valve with operator is within a few dollars or cheaper on ebay. Can take parts from new valve and use them on your valve and still have the operator. Valve needs to be powered down or turn thermostat way down for a few minutes head twists off. To get to screws.