2000 Dodge Caravan noise when heater in on

Hello bob and ken,
this is with regards to the noise that I hear everytime the heater is turned on. The noise is coming in the center or cabin behind the radio and in the glovebox. I tried to turn the selector switch and the noise comes off only on the winshield defogger. What must be the problem? And I another question with the same vehicle. The check engine went on and the code is P420. Is it the oxygen sensor or the MAP sensor? Hope to hear from you and thanks again for the great help you are providing us.