Thanks everyone sounds like I'll take a look at the rocker arms for wear and see if there is a bit off wear. Another winter project

Response to 52 Carl if what you say is happening would I notice the engine start to run rough because all the valves not closing properly? The truck has no lose of power when this happens and no difference from when there chattering.

Response to bartamos the truck runs at 160 all the time once up to temp.

Thanks again for your comment s and suggestion s
Kevin