2005 Impala making helicopter noise, even when off PLEASE HELP!!

Hello, I have a 2005 Chevy Impala and I got the oil changed on 7/24. On 7/30 I got into the car and before I even put the key in the ignition it started making a sound like a helicopter. It is very loud and it sounds like it's coming from the center of the dash (where the AC controls are). I turned the car on and it continued. Ever since then it has done this on and off. It will eventually stop but sometimes it starts again. Sometimes I notice that it starts when I hit the button on my wireless remote to unlock the car and disarm the alarm. Yesterday it did it while I was driving but stopped before I got home. I was in the house and my brother came and he said when he was outside he heard it and it was coming from the passenger side front, near the windshield. I am baffled by this and hope you can help. The thing I think is the strangest is that it does it even when the car is off.

Some of the Monte Carlos and Impalas have a problem where the blower motor will not shut off, even with the keys out of the ignition... That is most likely what is causing the sound you hear... Check to see if you feel air coming out of the vents next time it happens.. I could be wrong, but it is a damn good starting point... Good luck =)

Thanks for the help, I'll check into it.