This appears to be cold weather and cold start related. I seem to recall that this could be caused from a loose heat shield (but searching did not yield the post)? and was not belt related? The car has 65K on it so I am not above replacing the belt but it appears to be like new. Goes away withing about 300 feet from my garage but will do it at idle for a minute or so. Does not return after i drive it a bit, until the next cold morning start?

 

Check the tensioner pulley