What clatters on the roof
with a quick, impatient hoof?
I think it must be Santa Claus,
dear old Santa Claus.
Then he remembered recognizing that very tune when he'd heard this work on the radio, which the announcer then identified. Thus Bernstein was able to correctly say to Maestro Reiner, "Yes, I know this piece. It's the Academic Festival Overture by Brahms," and thus got into Reiner's class.
And here it is.