I should point out that the frequency response of the microphone on my wife's smart phone really falls off below 100Hz, so you need to be careful to not read too much into the really low frequency energy if you try this. I actually picked up a noticeable drone, only at idle in gear with the UF+MF. It doesn't even show on a plot, but my ear doesn't lie! As soon as I crack the throttle it goes away though.