barnaj

Hey guys looking for some help. Bought a 2006 Monte Carlo SS, certified used from Chevy. Had the same exact car when they were new and had no problems whatsoever. Since ive had the car i have this strange rattle/grind/whatever from the passenger side. Its so hard to explain, kinda reminds me of a computer when the hard drive is working really hard. Had it into chevy 3 times for it and they cant seem to replicate it. Its actually in the dealer right now again. It is so odd. It doesnt do it til you shift into about 30-40 mph range. If you take foot off the gas and speed hits 12-10 mph it goes away like clock work. I had some flowmaster 44's put on, so had the exhaust checked, they found nothing loose. At first thought it was something in the vent, had that checked, found nothing wrong. Wondering if it is alternator or belt making noise that is reverberating into cabin. Also maybe that cylindrical white piece under the cabin air filter? Ive been racking my brain trying to give them things to look at. Sound comes from interior passenger front. Its driving me absolutely crazy now cuz im fixated on it even when other cant seem to hear it!! The dash, panels, glovebox, vents, everything is tight so it MUST be coming from under hood or inside passenger door. Has anyone had this problem or have ANY ideas for me. Im pulling my hair out!
Thanks.