1998 Oldsmobile Intrigue Question: Loud noise/vibration when using heater or air conditioner

When I use either the air conditioner or heater I get a very loud vibration/noise. It still heats or cools, but so loud it can be heard outside of the car. What is this? I tend not to use it except for a minute or two at a time, then turn it off. This happens EVERYtime I use either and starts within 15 seconds of turning them on.