If a rattle and not a machine gun type noise:It may just be the changing momentum of the water making the pipes move in the wall. Strap them if possible, or use a can of foam to secure them. Sometimes just a more restrictive faucet aerator and toilet ballcock will slow the water down enough to stop the rattle. Or just forgt it and see if the new buyer complains, then let him/her have a few dollars off the asking price for repairs.If machine gun type noise… Change the loose faucet bibb washer or wornout toilet ballcock.