How's it actually driving in general? Maybe brake related with that kind of noise, pads/shoes sticking a little would cause such a noise, particularly in this weird weather! Though that would be noticable in the feel of it when you put your foot down.
If it's most noticable when in neutral rolling down hills then maybe it's just part of the aero's on the car causing the noise?!
main things I can point out is the noise is definatly low decibles and its only there when between 37-43 mph, this tells me it cant be anything to do with the axels or im sure it would just gets worse at higher speeds. I think the best thing I can do it go down a hill and turn the engine off.
If the noise is still there its definatly not the engine.e
When i have the windows up and I can hear it its coming from close to me or just behind me ( but saying that I never sat in the passanger seta, maybe thats worth a try ). I recently had the wheel off my side front.
When I acellerate past 41mph ish the noise starts to quietent down and drown out, Same as if Im doing 40 and I slow down using the brakes it drowns out and doesnt just cut out but quietens down then becomes nothing and non exsistant
Its the most faint sound I can hear just above the engine and not boasting again but probably due to my hearing or I wouldnt hear it.