UPDATE!
Well, I thought given the weight of the 986/996 knob and the silicone that jammed into the knob before mounting I would not have any buzzing or rattle... Not so. In the lower gears, buzzes a lot. Higher gears not an issue. Accelerating in lower gears causes buzzing as well.
The issue is the shift rod coming into contact with the inside wall of the shift knob. The 986/996 knob mounts via one set screw at the base of the knob. In the 968 knob, there is the rubber "cap" that sit on the shift lever plus the "clip" at the base. This effectively isolated the metal shift lever from the knob in the 968 situation. The 986/996 doesn't inherently do this. So, I need to isolate the upper part of the lever from the inside wall of the 968/996 knob.
I'll try some of the other methods which other folks have tried- lining the lever with felt/rubber/something plus I will also try to fab up some sort of "bushing" or isolator for the set screw to help absorb vibration.
Bottom line- the buzzing & vibration is unacceptable to me and I need to either severely reduce it or eliminate it completely.
We'll see...