check for loose belt pulley at crankshaft,i had one that acted as you describe and did not find the source of the noise until it got worse and by then the bolt holes were worn badly.I had to replace the pulley(dealer had to get it from Germany) and all the bolts

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Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles