hey second post to this .... 1999 F150 triton 5.4, with 27k on clock.,.. since new has a loud metal flexing sound coming from cats,(while driving, every ten seconds) same sound as when shut off, cooling metal sound hot to cold... is this normal? has anyone had a FIX for this, dealer has clampped same with no noise reduction... Thought it was "piston slap," but told no several times... THANKS IF YOU CAN HELP..

 

Mine does this only after turned off. Freaked me out when I bought it because it is the loudest I've ever heard. I've seen others ask about this, so it must be common.

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
4-wheel disc/ABS,Chestnut/Parchment,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.

 

I too heard this, I picked up my truck w/ 238 miles on it. I put 200 miles on it in the 2 days i test drove it. At first it was very loud. Usu in the evenings when it was cool out side it was the loudest. W/ just 1300+ miles it seems to be non existant.
I knew what it was and it is just the metals expanding and contracting. In time they should do this less and less. Kind of like a house settling after it's built. My understanding is the first 5 years or so are the fastest and after that settling will normally cease.

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Dr.(wife says i'm spoiled) Jones.
5.4L(whatpowerawaitsthee)
SC4x4.
djones01@elp.rr.com