Toby --

I like Joe's answer -- the fiber timing gear can make noises, 'though it's often an intermittent hammering sound. And usually that's just before the teeth fall off.

Just behind the fiber gear is a brass retainer plate that secures the cam. If it wears, the cam might drift fore 'n aft a bit.

Also, there were a few years in there when the cams were soft, and the lobes would wear.

You should also think about wear in the rocker shaft, and of the pushrods, the rocker arms, and the adjusting screws. Look carefully at the ends of the rockers where they contact the valve stems.

As the valves, lifters, or pushrods rotate -- any uneven wear pattern will manifest itself as changing valve lash. If the rocker shaft is uneven or if the bores in the rockers are worn, any fore 'n aft motion of the rockers may change the clearance.

If the locknuts on the adjusters aren't socked down, expect them to wander.