hi i recently purchased a 99 gt 4.6 the guy i bought it from told me he had recently changed the rear gears to 4:10 the car feels smooth but there is a noise coming from the bottom on the rear it sounds like a rubing noise and i don't know what it could be i thought maybe it could be the diferential that doesn't have enough oil or i dont know what if anybody has any idea of whatr it could be please help me ouy thanks.    

is it a hum?  

could be a bearing went out or is going out if the shims are too tight.  I had the same damn thing happen to me.