The ceiling hvac register in my bedroom is noisy. The noise is
primarily broad band "whishing" from the air over the blades of the
damper and diffuser. If I remove the register, the duct is very quiet.

Are there quiet diffuser/registers out there? I just replaced the
register with one that has individual curved blades which helped a bit.
The damper portion is wide open and only contributes to noise making.
Are there diffusers withoug dampers? Would it be advisable to dampen
the line closer to the attic AC unit? Or getting a professional to
reduce the fan speed?

The room also has a return vent. Of course, it also is a noise maker.
I was able to stop the high frequency singing by "tuning" the grill
blades.

Thanks,
Jim