So, I took my bike back to the LBS to examine the issue I am having with my Sturmey Archer S-RF3 3-speed internal hub. He took it apart and said there is no evidence of grit, debris, misalignment or malfunctioning of the hub. After test riding it, he suggested it is normal for Sturmey Archer internal hub to exhibit intermittent vibration/rumble when coasting and for some resistance/noise when backpedaling. (He indicated he has a SA hub and it does the same thing.)

I suppose my follow-up question here (to other SA hub users) is, is this consistent with your experiences? This is my first SA hub, and as I said initially this wasn't an issue I noticed in the first six months of use (i.e. prior to servicing).