jtisdale,
Some HX40's are louder. It depends on the compressor housing. The one I got has no silencer ring and is not designed to have one.
The ones you get from Piers typically have an HX35 housing that has been machined to accommodate the larger pinwheel on the 40 and has the silencer ring. Or they will have a true HX40 housing but one set up with the silencer ring.
I'm getting a machined 35. Al says there will be no change in performance, just a quieter turbo.
-Jay