Do you ask because you might be putting money into an engine that's going to fail some other way or because you're putting money into a vehicle but not increasing its resale value? Either way I think it's time to throw money at a professional assessment.

My plan of attack would be to run the engine with the belt removed to take the ancillaries out of the picture. Then I'd check chain elongation. Actually, I'd check it anyway if it hasn't been done in the last year. While checking the chain, I'd remove the cam sprocket and post pictures for an assessment of wear, and remove the upper guide to make sure it's intact. Then I'd remove the vacuum pump for an inspection. I've reused metal gaskets without introducing leaks. Make sure the front cover of the vacuum pump is on tight. You mentioned a new pump. It has bolts on the front cover, right? Then I'd pull the side pan to look for debris in the sump. None of these tasks requires special tools or skills. The cam sprocket bolt is the only thing I'd torque to spec. The rest is torqued to feel. You'll need a side pan gasket for sure. The valve cover gasket and metal vacuum gasket can be reused. Of course there's the cost or resolving the cause of the noise if you find it.

Another thing to try is loosen each injector line one at a time to see if you can isolate the noise to a cylinder. A lot of questions remain but it acquits the chain. Hmm... Johnnie Cochran could have used that line

That's the extent of what I can do by way of an assessment. If that doesn't reveal the cause of the clatter, off it goes to an indy for his opinion.

Sixto
87 300D