Droning noise

[julezw]

Hi Folks,

I have searched and read a lot to no avail. So here is my issue:

A low humming droning type noise coming from below the car towards the engine bay. It is not very loud so recording it will be impossible and its hard for me to tell whether its from the engine bay or under the car, but most likely the latter.

This noise occurs at really low revs, low speed and with not much throttle. I notice it mostly when I slow down from speed and then creep up further gently on the throttle. I also notice it when going up hills very slowly.
I hope its not transmission related? I had a rebuild 40000 kms ago. Could it be belt related? Pumps?

Any input would be most appreciated!

cheers