This has been driving me nuts...why do _school_ buses sound different
as compared to any other type of vehicle? You can always tell when one
is coming, even if you can't see it (over the rise of a hill, for
example). Greyhound buses, RVs, motorhomes, dump trucks, semis -
school buses don't sound like any of those. It's been suggested that
the long exhaust pipe and/or long drive shaft may contribute, but why
wouldn't the other "long" vehicles sound like that? I'm sure you all
know the sound I'm referring to, it's a particular type of whine (gear
noise maybe?) that increases relative to the speed of the bus.

For what it's worth, I agree: School buses do have a unique signature.
I'm sure a researcher will be along shortly to answer definitively,
but to hazard an utterly ignorant guess: Might it be related to a
"governor" device that limits the vehicle's maximum speed? We were
told as kids that all school buses were equipped with these, and that
buses cannot exceed a certain speed. (Of course, we were also told
that swallowed chewing gum accumulated into a ball inside your
stomach...)

As something I regard as 'common knowledge', American school busses
are the safest form of transportion on Earth (as measured by
passenger-miles, total number of passengers plus total number of miles
traveled divided by total passengers (I think)). Commercial aviation
(airlines) are the second safest. I don't remember any noise
particular to school busses. All I know is you damn well better stop
behind them when the 'stop' is active, and I hope those who pass ahead
of me and the bus itself get caught by the cops get beaten soundly.

Not to detract from the future answer, but it is because they use
deisel engines.  Deisel engines use deisel fuel.  One reason some like
deisel is because it is so safe - it take an emmense pressure in order
to combust it.  I've never done this, but I'm under the impression
that if you lit a match to it, it wouldn't do much (explode like gas
would).  Many of the qualities of the engine have to do with the fuel.
 For example, pound for pound, I believe deisel is actually more
efficient.  It has more potential energy in one pound than does
regular gasoline that you buy.  So deisel engines do not have to turn
over nearly as many times in order to achieve the same power.  Their
RPM (revolutions per minute) are much lower than regular gasoline
engines.  Lots more torque, less horsepower

You're not the only one who cares about the sound of school buses.
Here's a discussion thread that considers various models and debates
which ones sound the best.

http://www.schoolbusfleet.com/forum/post.asp?method=ReplyQuote&REPLY_ID=75193&TOPIC_ID=10783&FORUM_ID=5
Scroll down to IC    Posted - 05/14/2005 : 10:40:03 PM for the start
of the conversation about school bus sounds.

I think the unique sound of school buses results from the relative
lack of insulation in comparison with other buses, both for the engine
and the passenger area.  The side panels, ceiling and floor vibrate
more freely.  And yes, the long open driveshaft contributes to the
unique sound.
Myoarin