Knocking/Tapping noise in 87 300D

Hello All,
As of late, my 1987 300D Turbo has developed a bit of a tapping or knocking noise that's only really audible at idle, not quite on every cylinder firing
but it's very steady and noticeable. It used to be a few seconds on, a few seconds off, but now it's consistant at idle. I recently did an oil, oil filter, and fuel pre-filter (the small plastic rough screen filter) change but was unable to locate a new canister fuel filter. Could this tapping noise be water in the fuel filter (which I am going to replace tomorrow), or possibly some erroneious lifter noise with the new lighter weight oil (went from generic 20w50 to Delo 400 15w30)?
Any advice greatly appreciated! Mike
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This is a 3.0L straight six? If so, its hydraulic valve compensators are known to tap. Lighter weight SAE oil may have caused the tap. Synthetic oil may eliminate it.
Water in the fuel filter makes NO noise but it may cause the paper filter element to swell shut.
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Correct... 3.0l straight six. I don't understand why the valve compensators would start tapping several weeks and 1500+ miles after the oil change? Why were they intermettant and now solid?
I plan on running 20w50 Amsoil Synthetic through the ole girl as soon as I can get the other cost-intensive things about the car in line. This tapping doesn't sound like normal diesel rattle, which has me a bit alarmed.
Any other help appreciated! Mike

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