Car: 1989 260E

I just returned from a trip for the weekend. While leaving the airport parking garage, I noticed a tapping noise up front, identical to what I would sometimes hear for 2 or 3 seconds after starting the car in cold weather. This time, however, the tapping does not stop. The sound precisely varies in volume and frequency with engine speed, and it visually matches the frequency of rocker arm movement, as I observed when I removed the oil filler cap and looked inside.

Looking under the hood, the noise is definitely from the front of the engine, and definitely a continuation of the tapping I mentioned upon cold weather starting. I checked the belt tensioner shock and that is not the culprit. I pulled the first three (front) spark plug wires in sequence and the tapping persists. Also, the car shakes noticeably, as if one cylinder is misfiring, and the oil pressure indicator needle is very active -- moving is a somewhat erratic manner.

The only work/additions to the engine in the past few weeks were: 1.) New injector seals 2.) Lucas Oil additive to engine (I detect a possible connection there).

The low oil light did come on briefly a few days ago, and I added about 2 quarts then, but upon checking the level am coming up a bit short and have since added 2 more.

If I reove the valve cover and have a look, would any problem area be obvious? I am scheduled to go to the mechanic's on Wednesday morning, and don't want to mess with anything unnecessarily.

Any thoughts or suggestions would be appreciated.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black