I don’t know if this is oing to be any help to you but I drive an 09 plate 8 wheel Scanny tipper and mine has a what can only be described as a vibrating / whooing noise.
My mate who I work with had his new one a year before me (I had a 55 plate at the time) and I remember following him down the motorway for the 1st time…same droning noise i’m getting from my new one.
The only thing I can put it down to is the tyres…my 55 plate had different tyres to both the new ones and it never made the noise.
Obviously with yours being a unit if it is the tyres your going to hear it louder as theyre only a few feet behind you!!!
I can still hear mine doing it, but not as loud as the rear tyres are quite a way back on an 8 wheeler!!!
Try and follow a new one…you’ll hear it with your window closed!!
I’m pretty sure mine has Michelin tyres fitted, but i’ll check tomorrow and let you know…
Hope this helps,
Cheers,
Roj