Ground locations...

[Chamberpains]

That sounds like the speed sensor is to close to the ring. Maybe a very small shim to give it a little extra clearance?
Also can the pick-up ring (tone ring) become magnetized from the magnetic speed sensor? Maybe causing this issue. If so can they be demagnetized?
This also a question I've had for awhile, because your not the only 1 to have this problem. But we used to ignore it because who cares, it's only at an idle. But these wonderful ELD's are forcing more work upon us so we need to address these who gives a damn issues now. Thanks Uncle Sam, I was having that "I don't have enough to do" feeling!!