I experienced the same thing in my 84 300SD about a year ago. It definitely sounded like a ping or nailing even over the usual "clack-clack" of the engine. I suspected a dirty injector and put in Red Line Diesel Catalyst. It's a synthetic oil and injector cleaner. After 5 min running with Redline in the tank, the nailing went away. It was probably due to the lubricating properties of the syn. oil. The problem came back on the next tank of fresh diesel. I figured the injector(s) must be really dirty or worn. I pulled the injectors out and tore them down. 4 of the 5 looked clean as a whistle and the pintle (or needle) inside the injector moved freely. 1 though was siezing up and when I managed to pull the pintle out, it was blue. I suspect water in the fuel combined with low sulpher diesel caused the injector to overheat from lack of lubrication. I ended up just buying a set of rebuilt bosch injectors for $45 each. I thought it wasn't that much considering I put on 260,000 miles on the 1st set of injectors.
In you case, I'd try Redline to try to clean/lubricate the injectors. I use Redline on a regular basis for the lubricating properties. One bottle is good for 100 gallons. Hope this helps...