Highway engine noise with echo (4)

I believe the answer is:

hume

I'm a little stuck... Click here to teach me more about this clue!

'highway engine' is the definition.
The definition suggests a singular noun which matches the answer.

'noise with echo' is the wordplay.
'noise' becomes 'hum' (hum is a kind of noise).
'with' says to put letters next to each other.
'echo' becomes 'e' (phonetic alphabet: alpha, bravo, charlie etc.).
'hum'+'e'='HUME'

Can you help me to learn more?

(Other definitions for hume that I've seen before include "Scottish philosopher" , "2 [PHILOSOPHER]" , "David ---, philosopher; Basil ---, late Cardinal" , "Nobel laureate" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.