1996 Oldsmobile Ciera Transmission Whirring Noise

167K miles on original 4T60E transmission. On cold start (overnite temp less than 50 deg) transmission makes "whirring" noise for about 2 seconds in park. Then makes same noise when engage any gear. Shifts seem normal (no slipping or delay) but makes same noise first few times. After a little driving to warm up, operation is perfect. Fluid level before cold start is up to bend in dip stick. Immediately after start is at lower end of temp range. When hot, is at upper end of temp range. Fluid and filter changed at 60K and 120 K. Problem started over a year ago after I accidentally let vehicle idle for about 15 minutes with vacuum line disconnected from modulator. Lucas transmission additive fixed problem until now. Any suggestions short of complete overhaul? Tx for your advice.