Nailing sounds like a hammer nailing a nail, it has a crisp sharp sound to it. Bring a 17mm wrench with you and break each line loose, if you find the one which quiets it down then it is quite probable that cylinder needs a new injector.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k