Well like the previous post stated, there could be arcing of the spark from a bad plug wire to somewhere other than the sparkplug, but you would notice a constant miss while revving the engine. The other thing that could be making the noise is a vavle clearance gone big.
The overhead cams do have close valve clearances and aren't really noisy when running, but if one starts to get more clearance then the clicking gets louder. The only way to really tell is to measure the gap between the cam and the valve, with the help of your maintenance book. If one or more are 4 or 5 thousandths off then they will be noisy and need to be reshimmed.
The oil can amplify or deaden the noise some depending on the age and viscosity of the oil. If the oil is old and thin then the dampening qualities won't be there and noise will be louder. Change the oil and put a good 40 weight oil in and listen to the noise to see if it has gone just a bit quieter. The thicker oil will cause a better film between the cam and shim and it should get quieter.
The oil is just a splash fed system on top and once the oil is up there it is lubes the cam, buckets, valve stems. Thinner won't help the noise, especially on an older engine. Clearances get larger and require more oil to fill the gaps and thicker oil or a good additive helps there.
You have other things making noises on top too. The fuel injectors are constantly opening and closing. The distributor will make a small amount of noise too. You may even have a piece of the timing belt coming off and hitting its cover.
In the extreme you could have worn out pistons and they or the wrist pins could be making noise with again too much clearance.
I know this was to vague, but maybe one of those areas mentioned might be the culprit.
Let us know.
Russ
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