Boiler off...knock in radiator?

I had issue with boiler leaking water last winter which turned out to be ancient expansion tank. Prior to that, I had some knocking.  Not much.
Today, I’m sitting in our house. It’s summer so at times house is hot, though I do have room air conditioners on first floor, used occasionally.
The radiator knocked!  The boiler is off!
Anyone have any idea why it would knock now?  Should I worry?
Thank you.