Is he complaining about the sound of the water running through the pipe, or gulping air, or what?
Assuming it is done in plastic (PVC or abs?) then packing some insulation around the pipe will help some. People that are used to cast iron drain lines don't realize how noisy plastic can be.
If it is gulping or something else, it may not have a proper vent line or the drain line connections are not done properly and water running by that connection is creating problems.