It is normal depending on your gear set.
Mine does it and I know a lot of people's who does it as well.
if it was made by motive it does it pretty much does it too.
dont let ppl scare u online here telling you it was installed improperly, most people with 4:10's on the S197's rears make some humming esp on Deceleration that I know pretty much of all of south florida.
i forgot the f'n reasoning something w/ the gear being really small and its spinning really fast and some other ****, i cant remember either way ur fine. u either deal with u or keep trying to find a gear set that doesnt make the hum, but because it makes the hum does not mean their is a mech. problem.