What causes a whining sound in the transmission of a 2002 buick rendevous when it is in gear. Any idea on cos?

The car has recently started to give a loud whining sound when in gear. I am trying to decide whether to keep it or dump it as is.What causes a whining sound in the transmission of a 2002 buick rendevous when it is in gear. Any idea on cos?It could be a bearing on the drive shaft but it could also be a lot of other things ,sorry not to be of any help .What causes a whining sound in the transmission of a 2002 buick rendevous when it is in gear. Any idea on cos?have you been changing fluid on schedual?? bearing wear out,,pumps fail,,fluid clean and full??