Add around 5% of fresh vegetable oil in your diesel. It won't harm your car and it will quieten it by quite a bit.

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1992 Mercedes 190D 2.5 turbo 5sp manual. EGT+boost gauges. Boost controller set to ~14.5 psi. 1 1/4 turns on full load adjustment. LPG injection. Next in the list is water injection.