Picked up a 2017 Denali just over a month ago, been hearing a "fluttering"/rattling type sound when accelerating. I'd say it usually happens at about 1500-1700 RPM... occurs when you're asking for a little more speed and usually stops when at cruising speed. Dealer said it's the sound of some exhaust valve with the purpose of keeping the muffler quiet... is this normal? Seems too loud to be normal. Video attached.