Drop your pan and replace your filter. I had a 4L80E that was purchased from a wrecking yard. Well after the motor rebuild, it was brought to my attention that the engine/tranny apparently had been submerged underwater once before. The tranny shifted fine in all gearsm just had this bothersome whine. Local tranny builder said it was a filter whine. Low and behold, dropped the pan, changed the filter and fluid, no more whine. Good Luck!!!!!
