rattlesnakes under the hood
 

Not literally, it just sort of sounds like it. It sounds like "diesel knock" ( bucket with a handful gravel in it) and occurs while accelerating and stops when you get over 40 mph or so . When you're cruising on the freeway at 60 mph or so ( which is about right in a 240D) it's quiet. Generally it goes quiet under full load with your foot to the floor. Thanks for any advice anyone may have. I just today got an '84 300SD so I'll be asking more questions about that.

rattlesnake update
 

Shortly after I posted, the noise went away. I have no idea why. I went to the auto parts store and got some magic diesel elixir ( the one in the grey bottle). I poured it into the tank in the store parking lot. When I drove away the noise went away. Cause and effect? I don't see how it could have worked that fast. There wasn't time for the stuff to mix with the fuel. The only other factor I can think of is that it was a rainy day. In gas engines water mist injection can stop pinging. Pretty far fetched it seems but otherwise a strange coincidence. This same noise has come and gone several times over the last couple years so I guess I can just hope it doesn't come back. I've tried to describe this phenomenon as accurately as possible. Anyone have any further theories?