I experienced the whistling of the XD-4 too. I got it to stop though I can't remember how. I think I fiddled with the tension of the mounts that hold the peak on. The peak always worked really good for me both as a true peak and as a sunshade if I kept it down about as far as it would go. I'd have to try to look almost straight up to get it to catch air. I wore that helmet almost daily for 5 1/2 years before deciding to head the expiration date. It was really comfortable for the first few hours but after a whole day I got a little pressure spot on my forehead. It was never bad enough to make me want to press foam in a little so I just lived with it. If it started to annoy me that was my queue to stop and take a break.
I've been riding in a Klim MX helmet since I trashed the XD-4 and it's pretty great but wanted a shield again so I ordered the Scorpion AT950. Hopefully it'll be pretty decent. I've always wanted modular helmet on long stretches of highway.... But after reading this thread and others while looking for new coms I think I might have ordered a size to small.
Which leads me to my question, for those with the Scorpion AT950 what coms work well with it? I'd like to fit the Cardo Packtalk Slim to it eventually. My old Sena disappeared somewhere between Keene and Alvarado a while back. Inspection revealed a small crack in the mount that prevented it from staying secured. I like the Cardo Slim because it looks like it spreads the weight around and looks like it could be tied off to the helmet with a short bit of fishing line to prevent total loss if it did come loose.