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So I had a question if anyone had this happen.
I re installed my cat back after having a straight pipe on my 01 LX and my cylinder head started making a horrible loud rattle from about 1500 rpm to 2500 rpm. Its runs and drives fine no loss of power but its super loud and it's getting worse over time. Can anyone help?

 
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Check your work, something might be loose.

 
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I did I even made sure my bolts are tight and looked around a bunch and wiggled and tapped things with a hammer to listen for that sound and when I rev up by the throttle body I can hear it coming from under the valve cover in the cylinder head itself

 
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afraid you will need to open valve cover to see
or you have a big crack on your exhaust header

 
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No it's not a exhaust leak I'm 100 percent sure it's a head noise