Hum from Heater

I have a 03 malibu and I noticed a hum from my heater today. When its on high it is really noticeable but when on low it isnt. Does anyone know what it could be? Nothing seems to be different. Still blows warm air and works fine. Thanks

sounds like something is in the cage of the blower motor. on the passenger side, under the dash is a bullet shaped cone. it MAY have a cover that you have to take off to see it. anyway its 3-4 small bolts. unplug the connector take out those bolts, drop the thing down and pull whatever is in the cage out. reverse for installation.