Humming noise from engine bay

[Drewby]

As above, I recently re-brushed my run on pump, and was hoping that the humming was that, but the noise only happens when the engine is running, and for about 2 seconds after the engine is switched off. I pulled out the rain cover at the back of the bay to check the pump and see if I could feel it vibrating which it didn't seem to be doing. So my question is what else in the engine bay makes this kind of noise whilst the engine is running and cuts out 2 seconds after the motor stops. As fad as I can tell the sound seems to be coming from near the vacuum pump area, would this be the noise?