Q: Blower motor slows down once I start driving and stays that way.

My a/c works just fine until I put it in drive and start driving it where it then trickles down to a whisper of air that you can barely feel on your hand. It always makes a constant noise when doing this. When I park the car or stop it's the same thing and if I turn it off, wait a few seconds and then turn it back on, same thing, but I went to the store and was inside for about 10 mins and when I came back out and started the car, full blast again, but once I put it in drive and accelerate, it goes back to making the sound with just a trickle of air coming out. The air is also cold the entire time.

My car has 160000 miles.
My car has an automatic transmission.