Sorry to be such a pest, but I have another question about our aging steam
radiator system in our six-unit apartment building. Last night, an hour or
so after the weekly flushing of the systerm, we heard a very loud knock in
the pipes. As if someone had hit the pipe with a sledgehammer.

Is this dangerous? What should I do? Was the knock, which we heard only
once, probably tied in with the flushing?