when braking it produces a certain unusual sound which does not show that u are braking but rather that something is wrong. because it is a to and fro sound like,. its like u are stepping on springs of a bed now and again, its like quiki! quiki! quiki!. When u press hard the sound does not come out

The time i was asking you, one mechanic took my car and made a thorough search of the problem. he replaced the front disc where we apply brakes, but now he observed that infact the sound was coming from the rear and not the front. he says that the front was ok but the rear one. he tried to do scheming of the rear drums but nothing improved.