Originally Posted by jbmando
The lower tension on the strings is flattening your neck enough to cause buzzing. If you want to use 9s, put them on and set the guitar up again. If you can get most of the buzz out with 10s you can do it with 9s, too.
This. Loosen your truss rod and it should take care of it. Put your eye at the headstock and sight down the neck, you want it to be slightly concave at the frets.