Unusual noise when in stealth mode

Hey Guys

I have a 2007 Prius with about 77,000 km on it. Lately I've noticed that the car would make an unusual noise sometimes when driving at around 50 - 60 km/hr in stealth mode, with slight accelerator pressure such that the energy monitor shows power flowing from the battery to the motor (ie - not gliding or regen braking).

The noise is a bit difficult to describe, it kind of sounds like "whaaa - whaaa - whaaa". It is not a very loud noise, but it is certainly noticeable even with the radio on. The frequency of the sound appears to be associated with speed, however the noise disappears if the car goes above around 60km/hr, or if I let off on the accelerator, or if the ICE kicks in. It is also difficult to reproduce this noise at will, it sort of just appears on its own, but the conditions for the noise to appear is almost always the same... 50 - 60 km/hr, with slight accelerator pressure.

Does anybody here have any guesses to what this noise is? I am worried that it could be a sign of a bearing about to fail in the transaxle.