My 2010 Xterra has 42K miles and is making a soft whining noise while in gear.

What is your question?

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.