What walgamuth said... post a recording of the sound.

Could be injector nailing... My 300D was nailing so loudly it sounded like a ball peen hammer hitting something. Two tanks of B99 and a set of fuel filters later, no more knock.

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1991 300D 2.5 "Rocinante"
2002 Golf TDI "Teen Spirit"
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1984 300D
1966 Mustang I6 3sp
1985 Mazda RX-7 GSLSE
1982 Toyota Supra
1977 Datsun 280z
1971 Datsun 240z