I remember in primary school, one of the buses we used to get driven around in for excursions, weekly swimming lessons, etc.
(local bus company)
It was super loud with the engine under a massive cover up the front next to the driver.
I never knew what it was at the time... but vividly remember the racket it made.
I've since learnt, it would've been a 2 stroke commer knocker engine or similar!