New radiator fan, the old B9TLs have a single, large radiator
fan that does not cover the whole radiator, the newer B9TLs have 6
smaller fans that cover most of the radiator, which helps dissipate
heat more effectively. A lot of the noise you hear from buses is
from the radiator fan.
Maintenance of buses by individual compaies play a part too.
This is how things should be - incremental mechanical
improvements.
You know something is really up when a manufacturer makes
aesthetic improvements to the product in batches rather than
mechnical ones, thinking that it is the pathway to secure that next
huge order.