I have a '91 F150 4X4 w/ 302 that lopes at idle warm or cold, and makes a sort of clacking noise under load. Sounds kind of like an exhaust leak on only one cylinder. Anyway, I'm hoping it might be something simple like replacing the plug wires and cap and rotor, but I'm worried about the valves. Anyone with more experience with this sort of thing have any ideas or suggestions?

 

Give her a good tune-up...spark plugs, wires, cap and rotor. Check your air filter, replace if dirty. Change your oil and run a quality synthetic like Mobil-1 and then see how it is. How many miles are on the truck? Explain more about the sound and exactly when it happens. If you have higher miles then it could be your rod bearings on their way out. As far as the loping idle...that should be fixed with the tune up, unless of course a previous owner had installed a hi-performance cam. Good luck.

 

What you describe sounds like pinging(detonation). I'd suggest the tune ups as was mentioned before, and try running a tank of premium or mid grade just to see if the noise disapears. If so, you know you need to run the higher octane gas.