I had this conversation a while ago with a bloke sitting next to me in the exit row of an EasyBus...
He reckoned it was related to the (apparently common) practice of taxiing almost to the threshold on only one engine "to save fuel", with the noise being the fuel pump working as the 2nd engine was started.
I thought is sounded like BS at the time, but was too polite to say anything but "Oh, how interesting..."