My 87 300SDL (190K miles) has recently developed a concerning noise which is clearly coming from inside the timing chain enclosure on the front of the engine.

The noise sets in only when the engine is at full operating temperature, and is a loud rapping noise that generally occurs during hard acceleration (though some times it's louder than others). I have owned this car for 8 years, and have never heard anything like it before. I know exactly where it's coming from since once the noise sets it, I can pull the car over and make it reoccur by revving the engine by hand with throttle linkeage while the hood is open.

I initially suspected the timing chain tensioner, though I have replaced it and the noise has not gone away. I now suspect that it must be one of the chain guides, but I don't know whether they have some kind of hydraulic actuator or not.

Has anyone had a similar experience they can share?

I'm assuming the repair required here will be to remove the timing chain cover on the front of the engine, which requires the fan hub and radiator to come out too -- I assume 6-8 hours of labor, plus replacement of whatever parts have failed inside. This is a 20 year old car, and I'm struggling with whether I can justify the cost.