I expect the reason your vibration diminishes after warmup is that the Balance Belt streches a bit after it warms up.
My car has a BB whine when it is cold. After 10 minutes it disappears. I have tried so many times, but never seen to get the exact tension to stop it, either the whine, or too loose (up to the point of slapping the cover!). I have just learned to accept it...no big deal.
As you can tell, I still think you have a BB issue.
Of course, this advise is worth exactly what you paid for it
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Bruce P.
2011 Cayman Base, LSD, Sport Chrono.
1988 951 totaled and missed
"You're the best I ever had" --JJ Cale