I believe a design concept and novelty on the 787 was no hydraulics (not sure about the brakes tho) . So if you were seated mid plane then upto about twen mins after take off things like landing gear and doors plus staged flap retraction could all mean something big and electrical has to do its work. No expert so dont take this as gospel but pretty sure the 78 has minimal hydraulics.
Perhaps this will end up like the barking dog on the A320 where the power transfer unit hydraulic pumps make an odd noise often when taxying