Hmm, GDD should be in acronyms. Is it Global Development Delay?
Please excuse my ignorance and not sure if I can help, but here's a few things we've tried - music, tickling/rough and tumble, a sensory space - dim lights, cushions, massage, and if all else fails, my dd responds with a louder noise - it sometimes shocks ds into silence! (briefly).
There are other threads and links about intensive interraction, part of which is vocal 'conversations' - where you kind of respond to their noise with a similar noise, but gradually bring it down a decibel or two, change it subtly to something less ear-bleeding, etc, which has worked for us.
Why is he doing it? Probably because he enjoys the noise and the fact that he's producing it. I appreciate that it drives you nuts.
I hope you get some more informed advice soon.