So if most manufacturers build their own engines, then I guess what we've been hearing on buses and the trucks we've mentioned is the disctinctive sound of Detroit Diesel (I know a few of the older Orions have Cummins 6 cylinder, but I don't know if I've ever heard those, and if they are anything like DD 6 cyl. I've heard the Cummins 4 cylinder thoughas there are several in Fresh Pond)
I forgot to mention that Crown school buses in southern California sound somewhat like city buses with the old transmission (and alot of them look like old-looks) There are many GM trucks that have the familiar 6 cyl. sound, and I think Autocar is another one with the 6 0r 8 cylinder sound.
The distinctions in car and regular truck engines is not as noticeable as the DD series. I've always noticed that a 4 cylinder car/van/motorcycle sounds alot like an 8 cylinder bus, and the new 4 cylinder (series 50) bus/fire/garbage truck is not too different from an 8 cylinder car/truck/school bus. A 6 cylinder car sounds a little like a 6V-92 bus, but once again, not as pronounced.