If "within the scrambled letters" doesn't mean "uses all the scrambled letters", but rather "uses only the scrambled letters" (plus the missing letter mentioned), then it could be:
with the missing letter being
the 2 Hs
The explanation would be
that it's a joke, based on the idea that engineers are so into engineering that they would rather listen to the sweet sound of a finely-tuned motor running well than anything that might more conventionally be called music, to the point where they would consider such a motor to be music.
(Note that I'm not claiming that that's my view of engineers!)