Haven't tried rolling down a hill with engine off, but noise is definitely there with engine idling rolling down a hill. When the car is moving, and noise occurs, I have put in the clutch to let the car idle while moving and the noise persists. Therefore, sound seemingly is related to car movement and independent of the engine RPMs. In the same situation, I'll rev the engine with clutch depressed and noise still persists unchanged.
I don't feel any abnormal vibration when it's happening.
Like I sais, I haven't tried rolling down a hill without the engine on, but based on all I have tried while driving, I am sure the noise would still be there.
Thanks guys for all!