Thanks for all of the ideas. I've not installed headers or an x-pipe, so we can rule that out. I'm also sure the weatherstripping isn't the issue, but you're right - I should silicone that anyway. I've also looked at the little rubber cones and they seem to be clean and tight.

I removed the HVAC blower fan itself from the housing. It looked ok and didn't rattle. I felt around in there as far as I could and felt nothing that wiggled. I also took out the cabin air filter and stuck a light in there through the screen. I didn't see anything metal that looked like it could make the noise I'm hearing. Also, the fact that it doesn't seem to matter if the recirc A/C button is pressed makes me doubt it has anything to do with anything in the air flow path. But I suppose there could still be something loose in the blower assembly.

At this point, I'm about out of ideas and hate to start disassembling the entire dash without having a clue what it is. I also hesitate to take it to a dealer for fear they will tear things apart and not find it either. If anyone has any other suggestions or things to try to determine the source, please let me know.

Thanks again!

 

Man, this is sounding like desperation time. Maybe use an automotive stethoscope or just a length of tubing with one end to your ear and the other end moving around in the general area of the noise to try to isolate the source.

 

It's definitely desperation time. The stethoscope might be a good idea. Since I've had my hand on everything that is reachable while riding in the passenger seat and couldn't stop it, it must be buried somewhere.

I've had and solved many rattles before (this is not my first Mustang...), but this one really has me stumped.

 

I have a very similar rattle, but it only starts with highway speed and after that it won't quit, I actually just put my car on stands to try to find it so if you figure it out please share, this is driving me up a wall.

 

Mine is not only at highway speed, but is definitely worse at highway speed. And still driving me up a wall, too. I've had multiple people listen to it and everyone seems to agree that it sounds like a thin metal plate rattling against another piece of metal. I still haven't taken it to a dealer because I'm afraid they will charge me a fortune and say they couldn't find the source...

 

Could the fender splash shield be loose?

 

I think it may just be a lose screw or something. I've had problems with my side mirrors on my 2011 because they're always loose and rattling. Not the same scenario but maybe it's just a problem with mustangs?

 

I don't know if you checked this, but I had the same problem as you a couple of weeks back - I was pulling my hair out on it! Turns out, the problem was a this metal Nevada license plate that said "SEXY" that I got from a Las Vegas gift shop that I inserted on the passenger side dash windshield.. and it was rattling over every bump.

 

I was really excited when I saw this post and I checked and found one of the fasteners missing. When I tapped on the splash shield, it tapped the fender and made almost the same type of sound. So I got some fasteners and replaced the one and still heard the rattle. Then I found that if I tapped on the splash shield, I could still get it to move away from the lip on the fender and make a noise. So I got some 3M double sided foam tape and put it between the splash shield and the fender and there is no longer any noise when I tap on it. ...but the rattle is still there. So what seemed promising turned out to not be the problem. But thanks for the idea.

 

I don't have a license plate on the dash, but that's about what it sounds like. After running through the exercise of trying to quiet the fender splash shield, I really think the sound is coming from inside the car. I think the next step may be to remove the airbag. I've heard of a "plate" attached to the airbag that can rattle, but I'm not sure what people mean by that. I can touch the airbag cover plate on the dash and not affect the rattle, so it's not that, but maybe there is something behind that could rattle?