(Stupid question...) That would imply a sound coming from the rear of the car, right? Although I know it's possible to get fooled on locating weird sounds, this really sounds like it's coming from the front, specifically the passenger side.
In any case, having discovered that 3 of 4 axle boots were torn, I had the axles repacked and new boots put on just over a year ago. I haven't looked at them recently but it seems unlikely they should be a problem already.
