Probably a valve adjustment - those self-locking nuts can work loose. 
 
Do you know how to adjust the valves on one of those engines? It's not all that hard, but kind of messy. 

Richard

 Here's the GM way... there are a half-dozen different methods old Chevy buffs use, but this is the best in-writing way of doing it that I've found.

Engine Not Running

1. Start engine and allow to reach normal operating temperature, then shut engine off.

2. With engine in position to fire No. 1 cylinder, adjust the following valves:  Exhaust Nos. 1, 3, 4 and 8; Intake Nos. 1, 2, 5 and 7. To properly

adjust valves, tighten adjusting nut until all lash is eliminated, then tighten nut the specified additional number of turns. On V8-454 / 7.4L

engine, tighten nut 3/4 additional turn from zero lash. On all others, tighten nut one full turn from zero lash.

3. Turn crankshaft one complete revolution, which will bring engine in position to fire cylinders 4(V6) or 6(Inline 6 and V8). With engine in this

position, adjust the following valves: Exhaust Nos. 2, 5, 6 and 7; Intake Nos. 3, 4, 6 and 8. When zero lash is obtained, tighten adjusting nuts

as specified in step 2.

ENGINE RUNNING ADJUSTMENT

NOTE: The following procedure, performed with the engine running, should be done only in case readjustment is required.

1. After engine has been warmed to operating temperature, remove valve cover and install a new valve cover gasket.

2. With engine running at idle speed, back off valve rocker arm nut until rocker arm starts to clatter.

3. Turn rocker arm nut down slowly until the clatter just stops. This is the zero lash position.

4. Turn nut down 1/4 additional turn and pause 10 seconds until engine runs smoothly. Repeat additional 1/4 turns, pausing 10 seconds each

time, until nut has been turned down the specified number of turns from the zero lash position. This preload adjustment must be done

slowly to allow the lifter to adjust itself to prevent the possibility of interference between the intake valve head and top of piston,

which might result in internal damage and/or bent pushrods. Noisy lifters should be replaced.