Rattle at highway speed driving me INSANE

[EVL DEAD ASH]

Hello. New to this forum. Bought a 2013 GMC Sierra SLE a week and a half ago. A few days ago the truck started rattling at highway speed (rattle starts at about 55mph). As best as I can tell, the rattle is coming from the lower driver's side area, around the front wheel well, inside the engine compartment, or even underneath the truck. The sound will lessen or go away completely if I pull close behind a semi, but comes right back once I pull back out into the air. It sounds like heavy paper or a piece of plastic flapping rapidly. I've searched the forums praying to find an answer. I've checked the plastic covers that line the wheel well, the rubber seal under the hood next to the windshield, the windshield seals, and the front dash panel under the front windshield, as all of those seemed to be culprits for other people, all to no effect. Hopefully someone here has had the same problem and knows how to make it stop!!!! I hoped to attach the video I made, but it's too big. I uploaded it to YouTube instead. Link is posted below. I basically stuck my phone below the steering wheel next to the driver's door and recorded the sound. Any help is appreciated!

 

 

[EVL DEAD ASH]

Bump

[Chevy Joe]

That's a really strange sound, it sounds like some part is moving in the wind (like you said getting behind semi stops sound) wind related body part?

[muddkatt]

Do you have a bug deflector or vent visors on the truck?

[00Silverado4x4]

Not sure the exact design of the front end but behind the bumper there may be a deflector or air director that keep the air flow going through the radiator. I'd check to see if that by chance is loose or damaged. I worked on a friends truck, although it was the older body style, that was making similar noise and that deflector was just about falling off his truck.

[EVL DEAD ASH]

No visors or bug shield installed. I Spot checked the air deflector underneath, but I'll get under there and check it better. And yes when you pull up on a semi the noise lessens or stops, which leads me to believe that something is getting blown around. Thanks for the suggestions guys.

[EVL DEAD ASH]

Just really weird. Of course I have to listen while driving down the highway, but to me the sounds seems to be coming from something inside the front fender panel

[Mike-1000]

I'd check the air dam under the bumper then the mud flaps (if you have them) and then the inner fender liner.

 

Mike

[dreisenauer]

I had similar rattles in those places. 2 things that caused it for me: 1. mud flaps would sometimes catch air and vibrate/rattle 2. E-brake cable. Work it a few times and see if it goes away. I had my dash ripped apart several times chasing it until someone told me about the e-brake cable.

[EVL DEAD ASH]

BOOM!!!!!!!! It was the drivers side mudflap. There is a bolt and clip that secures the mudflap to the undercarriage of the truck. Someone cross threaded the crap out of that bolt and stripped both the bolt and nut. It was hanging on literally by one thread. I replaced it with a Home Depot bolt, washers, and lock washer until I can get a new bolt/ clip assembly from the dealer. Problem solved. Rattle gone. Thanks for all the suggestions and advice guys!!