I still get feedback on occasion when using FaceTime on OS X. If the call goes on long enough there is nearly 100% chance of this feedback occurring. Its very frustrating with Apple having such a tight ecosystem and being forced to use 3rd party programs (Skype) for a satisfactory user experience.
I thought it might just be my Mac but I can get it to happen on 2 iMac's and a MacBook Air. I think it has something to do with the data connection, I feel that the worse the connection is the more likely it will start happening. Once it starts it won't stop until you disconnect and reconnect.