Try switching to synthetic oil, if it doesn't help then you will most likely need to replace the lifters as required.

The 601, 602 and 603 engines came with problematic lifters from factory. I've got the bulliten fron 1989 somewhere around here.

I replaced all the lifters in my 602 and I haven't heard them since even with 0w40 mobil 1.

__________________
Alex.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32