Late last year the franchise for the 100 bus service was renewed and the
older buses were replaced by new Enviro 200Ds (I believe). These
obviously look a lot fresher and in general are quieter and don't suffer
the rattles of some of the 10 year old previous models.

However I've noted a strange noise eminating from them - a rather spooky
howl.

It's not always noticeable that it comes from the buses - some of the
time it just sounds like the wind blowing strongly - but seems to be
when they are under heavy acceleration. It's rather distinctive and
I've noticed it when I've been out and about on the 100 route and can
instantly identify that a 100 is around.

Has anyone else noticed this, and if so what causes it given that in
general buses are getting quieter?