Loud knocking in a Taco 222 Universal Flochek valve

Hi, We have a 28 year old Burnham I-6 system, and have lived in our house for 6 years. It's hot water with radiators. This year for the first time there is a steady and loud knocking that seems to be coming from valve above the boiler. The knocking sound like a large metal ball is in there, and resonates throughout the whole house. It only happens when the boiler is running, about every 5 to 10 seconds it will clang 3 or 4 times. A plumber came out and agreed that the sound was coming from the valve. He said he would have to come back and drain the whole system to replace the valve as there is no shutoff. Does that seem reasonable? I only ask because when I researched this issue, a lot of people said it only happens if there is air in the system, and that valve is not likely to fail. The heat seems to be working fine, by the way. It's more of a nuisance because I am a light sleeper and it wakes me up. Happy to answer any questions you may have. Thanks