I have a 1998 1500 with a 5.9L and it starts to make a ticking noise at around 40 mph and gets louder the more stress the engine is under(going up hills or accelerating faster) and sometimes the cylinder 8 will misfire but not every time and the code will go away a couple hours later. I have replaced spark plugs and wires as well as the distributor and cap. Any suggestions?

 

Checked the Plenum?

Does it sound live valve noise, spark knock? Is it indeed coming from the engine?

 

I have not replaced the plenum but the truck has a brand new motor and transmission with under 20,000 miles on it and the mechanic I taken it too said that there was no vacuum or gasket leaks but I checked the plenum thread and it basically has all the symptoms of it.

 

Check the plenum then. Also checked for a clogged cat.