I'd stop running it immediately and and tow it to a mechanic to have it looked at. You may have a timing chain jumped off time by one tooth on the cam gear. If it's still running at all, you haven't bent any valves yet, but if it jumps another tooth you'll bend valves and necessitate removal of the heads (much much more $).
If the timing chain guide rails are old, have them replaced! They are a common cause of timing chain failures in these motors.