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I'm old enough now, and had enough time invested in repairs and so forth, to believe I recognize a Heat Shield's rattle, when I hear it.
At least that's what I THINK I hear.......
Under load (up hills or when the torque increases) I hear a sympathetic rattle (or buzzing) coming from somewhere underneath.
My guess: one of the Heat Shields has a few tack welds that have popped on my 2009 F-150, and buzz (heat shield against the Catalytic body or whatever) when the sympathetic vibration hits a certain level.
All I really have to do is find it, to affirm what I believe.
Fixing it............well...........that's another matter. I suppose some garage somewhere will tell me that I should replace the component that has the Heat Shield attached to it, but...........I'd rather re-tack the Heat Shield or encourage that.
Many a heat shield have been fixed with large hose clamps. Wrap a couple of them around it and rock on down the road. It'll take two minutes and last longer then the original welds.
heat shield on my last vehicle (02' Pontiac grand prix) came loose and only rattled when idling in drive. I used some of my grandfather's cattle fence wire. wrapped it around the heat shield twice and twisted the ends together. lasted 3 years with no issue and traded it in with no one mentioning it.
I like the idea of hose clamps. Excellent thought!! Let's see..........I've not crawled under there yet but, what has a heat shield? The Catalytic Converter, at least. Any others on the 2009 F-150?
Heat shields are placed at various parts of the exhaust. Some are the thin perforated sheets of aluminum. While others are stamped steel or stainless steel. They can be found on the firewall of the cab, around the cats, there is one on the underside of the bed floor just above the exhaust, and one around the right side of the spare tire. And there may even be one around the fuel tank.
You may want to look into vacuum related issues in conjunction with the IWEs. A loss of vacuum, like when you're accelerating, will cause the IWEs to attempt to slightly engage thus causing what sounds like a rattle. What it's doing is eating off the ends of the teeth as they barely engage, or attempt to rather. This sounds like rocks in a tin can. If it goes away when you get out of the throttle, then I'd start looking there and quick.
This is all assuming you have a 4WD. If it's 2WD then good luck with your search.
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