Researching past threads on engine vibrations brought up the issue of proper propeller indexing. I found the article on page 45 of the 170 Book regarding service Information Summary on advancing the prop 45 degrees. I checked and sure enough, my prop was installed in the old location, trailing one bolt hole of TDC. My serial number fell before the the factory made the change. I have had the prop off once before and can not recall if it was installed differently before.
In January I had done a dynamic balance and added a fair amount of weight to balance the prop, so I was skeptical of how much improvement I would find in moving the prop forward the requisite 45 degrees. I assumed that any imbalance resulting from the improper position would have been compensated for by the dynamic balance.
Today I re-installed the prop in the "new" position and removed the weights added during the January balance job. To my surprise and pleasure, the low frequency vibration was gone and things ran much smother. I still need to do another dynamic balance to sweeten things up, but the prop is much happier (as am I) in the new position.
Bruce