There is no slip while changing gears, but while in drive at a stop and sometimes while mobile, I will hear this whining noise. When in park and sometimes at a complete stop it goes away. Someone recommended transmission filter/fluid change. What is the average price to have this done so I don't get bamboozled. Thank you.

"Gently" used vehicle, no towing or off road use.

The vehicle has 45k miles under normal usage (not off road) and my mechanic is recommending a differential oil change.The Owner's Manual is silent on this service. Does this seem appropriate? Many thanks.

At a stop light my heat-air coming out of the interior vents go cold and warms back up when I start moving again. The Heat gauge is in normal operating range and coolant level is normal.