Hi there,
with out hearing this noise it is a bit hard to say, but generally a bearing will hum, and get louder with road speed, diff bearings will alter pitch when you come off and on the throttle while in gear, a tooth noise is far more clattery or a coarse type noise, get a mechanic to listen and report his thoughts.
Mark (mhpautos)
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Sunday, August 9th, 2009 AT 10:51 PM