higher rpms / slight "chatter"
 

All this occured within the last couple months. (in this order) ANY THOUGHTS???

1) i noticed a little "chatter" in the transmission.
2) ford dealer flushed and changed transmission fluid. (their best "transmission person" observed no problems)
3) weeks later "a little"chatter returned.
4) aamco dealercomputer tested and road tested my vehicle. (he found nothing either).
5) i ignore the minor chatter; it often happens at about 40mph and about 57mph. (its really minor)
6) i just replaced my air filter. the vehicle now changes gears athigher RPMs, lower if i let off the gas. (coincidence?)

ANY IDEAS???