2004 Mercury Sable Humming noise

When my car is idling at a red light or when I take my foot off the gas pedal and the RPMs drop to idling spped there is a high pitched humming noise. I believe it is coming from inside the air filter assembly and could possibly be the mass air sensor. What do you think? Is this sensor easy to replace or will I need a mechanic to do the job?

Thank you,

Paul Mitchell