1997 Mercury Sable noise when I shift into gear

Greetings,

I have a 1997 mercury sable with 150, 00 miles on it. This vehicle runs and shifts smoothly but lately when I first start the vehicle, after it is shifted into gear, it often makes a brief sucking, whining noise that appears to come from the rear passengers side. It only does this when the vehicle has been sitting for some time. My transmission fluid is full. My mechanic has not noticed any problems and has not been able to reproduce the noise. It only does this reliably after sitting over night. Is it possible there is air in the system and a transmission fluid change could correct this? I do have a servo cover leak on the transmission. Its only a few drops a month. I haven't noticed this vehicle uses or leaks oil. I don't have any other fluid leaks. Thank you.