When the bearing seizes on a vacuum pump or falls apart the cam roller is then touching directly with the 2 arms of the armature where the bearing use to be , when this wears through this piece of metal you’ll end in a broken chain as it falls into this area .I Caught a failed bearing on a vacuum pump , the bearings and casing I had to fish out of the oil pan.

Is your valves adjusted?

I might take the accessory belts off first to see if the noise is still present. Noises appearing on occasion to be really distant from the suspected area can also occur. I also believe you can run the engine with the vacuum pump off temporarily. So if you pull it to inspect it considerer running the engine as well with it off. As a final test of it.

From limited past experience over the years. Alternators are guilty of this type of thing. Of course not always but seldom should they be overlooked as a possibility.

Noises have such a distinct individual sound. That if not easily resolved. If you can find a really older mechanic. It may surprise you at their skill of identifying the source of noises they developed. . In the day they were not paid for diagnostic time. Just to repair the issue.

For example when I arrived in this area. There was an old time garage run by two brothers called the Burke brothers. They had skills in vehicle servicing that put them in a total class of their own. I image they had been in the vehicle repair field for at least sixty years each and where still going pretty strong back them.