Peter Wells wrote:

Thanks to those who replied constructively, I spoke to a couple of service
companies and found one who seemed to agree with the suggestions given here.
They came out last night, took the burner apart, cleaned everything (there
was a lot of what looked like chips of rust in it) and now it starts without
the boom.

And the lesson to be learned here is...you could have done this and
should do such an inspection and cleaning every fall before you
relight it for the season. If you find something you don't understand
or something doesn't "look" right, then you can call the service folk...