Thanks for all the replies.
I wasn't talking about the constant whining, which is the cargo doors. But this noise raises a lot of pax comments before the push back or after the engine shut down. Couple of times I was asked by whomever I was traveling with , and I didn't know the answer.
People look at you funny when you tell them you are a CFI and you don't know the answer to a question about an aircraft system.
But something is not clear to me.. Is this basically due to the activation and de-activation of the hydrolic systems? Because I don't see why you would need them on the ground.