Kinda sounds like a bad fuel injector. Crack the fuel injector lines one by one and rev the motor, and you'll narrow down which cylinder is producing the noise. Then swap that fuel injector to another cylinder, and re do the test. Your looking to see if the noise follows the fuel injector.

Oh, and start running it on diesel, you might be able to save the engine.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg