I have a 1977 300D with 319,000 miles (50K on the rebuilt engine) that
I have owned for about 6 months. I love the car!!

My problem is an engine noise that comes and goes intermittently. At
idle it makes a "chirping" sound like a metallic cricket. Sometimes
when I accelerate there is a different noise like a high-pitched
metallic scraping that flutters rapidly from loud to soft. At higher
speeds, the noise goes away. I've sprayed belt dressing on the fan
belts, but that does not seem to help. From the location, I suspect
that the source is a failing water pump, but I am not sure, because at
idle and with the hood up, there isn't much noise. Also, the noise
seems to be more common when the A/C is on.

Is this definitely the water pump, or should I suspect the A/C
compressor? What further diagnosis should I perform?

Also, are there any service recommendations if I replace a unit like
the pump or compressor, like replacing the belts?

Thanks,

Peter Hollings
Atlanta