I have a 98 ,4.6 4x4 35 K miles - When I start it up and drive under load I hear a rattle ,somthing like engine knock , but it disappears after the engine is warmed up . I t doesn't matter what the weather temp. is and it has do be in drive at about 2000 rpms . I can't reproduce it just revving it up in garage .

Has anyone had a simular experience or ideas??

 

98 4.6 with 88,000 miles. Mine did the same thing, but only when it's really cold out. I was up in Lansing Michigan until a few days ago, and that's when I noticed it. As long as I let the truck warm up, I never heard it. Now that I'm back home in Houston, it cranks up and runs with no noises under load. That could be cause it's 64 degrees, though,,,,98

 

I have the same thing on my 2001 4.2L. I think mine is an exhaust leak that only occurs when cold and under laod.

 

I did an oil change and motorcraft oil filter per spec., disconnected the neg. battery terminal for 20 mins. and no more rattle . Now I can't say what did it , but the old oil filter was a cheapee from a quick lube outfit, or it could have been resetting the computer by pulling the neg battery terminal .The result wasn't immediate ,it took the second time out and no more rattle .

 

it sounds like you solved the old cheapie filter bad anti drain back valve problem!

good show!