If it's a lifter you could hear this noise independent on what gear you are in. There is a oil leak test described on the mbusa service cdrom but it requires some special tool to be done.
I'm not sure if it helps to take of valve cover. If engine isn't running there isn't a oil pressure on the lifter and there would be clearance.
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'87 MBenz 190E 2,3 8vlv
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