Seriously, I like the idea! Now that you mention it, I think that changing the lenght of the stack will only change the pitch of the noise. Good deduction! It sounds like this idea will reduce the amount of air getting blowing across the pipe, and still allow air into the stack.
Chuck: Post back up with what ends up working (or what the Supply Houses end up telling you to do.) It would be good, all-around info to know.