Its 100% a roots blower. The reason it gets louder under load is "A" your opening the throttle and letting more air in which creates compression of air, hence this makes the rotors hot and they mesh more. "B" The sound carries back through the throttle and air filter which almost amplifies the sound. When you change to a cold cone intake the sound is louder, its down to the design of the cone and the sound insulating properties.
I own a celica 1zzfe which I have custom built an M45 blower on. Attached is the celica running NA with the blower turning, this is what your blower would sound like if it was only turning and not connected to your engine. This was fecking loud.
P1000045.flv - Video - Photobucket - Video and Image Hosting
Also just to show the engine as almost complete.
Just turning the blower over with your hand creates a whine as the lobes mesh! then when the expand they become louder and louder. Also worth nothing that the sound will find a way out. Hope this helps