These are expressly for Thor:
Question: How do you know when there's a trombone player at your door?
Answer: His hat says "Domino's Pizza"
Question: What is the definition of "gentleman"?
Answer: Somebody who knows how to play the trombone, but doesn't.
Question: How can you make a french horn sound like a trombone?
Answer: Take your hand out of the bell and miss all the notes.
My personal fave:
What's the difference between a bass trombone and a chain saw?
Vibrato, though you can minimize this difference by holding the chain saw very still.