Knocking Radiator in 120 yr. Old House

Lafayette

We put new hardwood flooring down last winter and reattached 2 radiators once the job was complete. One works just fine the other knocks. It never knocked before, in the 15yrs. I've owned this house. My plumber & I went thru the entire house to make sure all the air valves were zone accurate. We also pitched the knocking radiator, which made the knocking worse.

On my plumbers' suggestion, I contacted a company listing themselves as Boiler Specialists. They checked everything I had already checked and had no new suggestions. They said the knock wasn't that loud and I should learn to live with it!  It's loud. We can hear it throughout the entire house. It wakes all of us up each morning at 6am sharp.

BTW-The boiler runs great. It's not low on water & I even put a cleaner in it a few months ago.

Any ideas?