Valve train noise/ fan problem

[rob81]

1988 MB 190e 2.3. Starting to sound like a sewing machine. What is the most likely source of the noise -- lifters, rocker arms, cam, valve guides, etc. and how do I figure it out? If it is the lifters for instance, should I replace lifters only or should I change the rocker arms as well? Is this a do it yourself job or are special tools required?

Also the magnetic fan only seems to engage with the A/C on, otherwise it does not work causing the engine to run hot--I think. I know there are some sensors somewhere but I'm not sure where and I assume these could be the source of the problem as well as bad fan. Not sure how to go about this. Thanks for any help.