[FRiSson]

I hear and feel a mild "chuffing" noise and slight grind that changes with speed of the car.

I have ruled out brakes, since it occurs with or without the brakes on.

I hear it on smooth pavement, otherwise there is too much road and car noise to hear it well. I have not been able to duplicate it with a mechanic in the car.

I have ruled out transmission, since it occurs when the car in neutral and/or the clutch disengaged.

It might be tires, but the low-speed noise and sense of grinding kind of argues against that.

Could it be differential, LSD, bearings???