Solved my tap-tap-tap issue
 

Last week after a trip up north to Hearst Castle the W109 developed a tapping noise from under the hood. I thought it was a rocker coming loose but then I came across a comment Tom made on this forum about his 4.5 making a tapping noise that is due to an exhaust leak. Well sure enough when I checked under the car today I found the large rubber O ring that supports the front of the exhaust pipes broke. By chance I had an extra ring in my spares box (thanks eBay!) so I replaced it and I was able with lots of WD-40 and a 12 mm wrench bent 45 degs to tighten the 2 bolts on the drivers side manifold/downtube joint. Well luckily it took care of the noise. I'm glad I ran across Tom's remark or I would have been pulling the valve cover off!! Tom have you tried tightening these 2 bolts?

Yes, I know where it's at. The #6 cyl at the manifold and head. It was at the #5, and I thought maybe if I got all 8 studs on the mani it would help. But it didnt. So I tried putting 2 gaskets on the #5 and it just moved down to the #6. My manifold needs to be replaced. If I pull the injector for #6, it stops tapping.