Rattle going uphill..

I am hearing a pinging or rattling from the engine compartment, usually going uphill, or in fourth gear. Anyone suffered the same problem before?
What could this be?

The 85 2.5 4cyl...

Poor fuel quality. Use a higher grade of fuel may help.
Excess carbon in the combustion chamber can contribute to this. A combustion chamber cleaner before your next oil change many help.

Your ignition timing may be too far advanced.

Your EGR valve may be blocked with carbon or not working.