My truck seems to idle fine and smooth. The problem is when I am traveling 60+ MPH and start going up a hill the power weakens and it has a rattle that sounds like the old style timing chain lightly slapping like they used to when they had bad gas in them. It does the same thing at 60+ MPH when I accelerate to pass someone. Anyone have any ideas?
Thanks,
Chuck

PS The engine has been rebuilt and has about 30,000 miles on it.

 

I had the same issue, it ended up being the engine removal mount slapping against the valve cover. I'd check that, it solved my rattling.

 

Chuck what is the year and size of the engine?
That makes a difference in what it could be.