Humming sound from rear end under acceleration?

Hi

As the title says...

I've noticed lately that there is humming sound coming from the rear of the car during acceleration and also during normal cruise. It "almost" goes away when i release the gas and coast (still there but very faint can barely hear).

What could cause this? If the axle is low on oil why would the sound go away when I release the gas?

Thanks

Syed