I guess that is the term.
My boiler,oil feed,hotwater baseboard,started doing this a couple weeks ago.
Sounds like little hammers lightly tapping the pipes. It's not overheating. Thinking it could possibly be air, I changed the Taco auto air bleeder,although it was only a couple years old,but usually if there is air,it makes a whooshing/gurgling sound. Feed valve is working fine. Expansion tank new last year(good hollow sound). Relief is not blowing off.