rotational noise at 70+ mph

[70chevellemalibu]

hey guys my 70 chevelle is making a high speed rotational noise heres the specs on my car and the story:

It has full UMI suspension, 4 wheel discs, 350CI, 200r4, 12 bolt 373s and posi.

Transmission and 12 bolt is freshly rebuilt, front wheel bearings are also fresh, drive shaft balanced, all 4 wheels balanced.

At city speed i do not notice the rotational noise (maybe the exhaust tone is masking it?) but i don't think so. Once i reach 70+ MPH i get a waaa quiet waaa quiet waaa quiet waaa sound no vibrations seems as smooth as it can be for a 50 year old car but its not as smooth as my newer sierra. This noise is not a constant waaaaaaaaaa sound its rotational.

Anyways i don't remember the noise being there last year and now its bugging me as i think there is something wrong with the diff.

Could the noise be tire noise or air chopping through the wheel wells or something? has anyone else experienced this?

Can anyone suggest any tips? i had the car looked at by the shop who rebuilt the trans and diff and they said they couldn't replicate the noise and everything seems tight and fine with the car but i want the car to be silent and perfect.....