If you think there is a difference and something is not right then your probably correct! From what your saying everything sounds normal, I will see if my car does the same on my drive to work as I believe it does kick up during shifts......which is normal operation! If it did not your shifts would be quite harsh. My advise would be to get a cheep scantool from harbor freight, autozone, oriellys or sears! Very useful investment for about 100$. Being able to use live data is critical....stock gauges are just not that accurate. If your only concern is idle not falling fast enough and you have no misfires or rough running or engine lights I would not worry about it. You may possibly need a reprogramming by dealer but you may not see any difference! One last question, did this start after you installed k&n air filter? Most cold air intakes require a tune to perform correctly but I would not suspect that with just a filter replacement. Have you tried putting the stock filter back in and seeing if that makes a difference? What year is your mustang? Do you get a slight hesitation at around 2700rpm?