I recently bought a 300D Turbo that had a nailing sound. When I bought it I thought that the sound was misadjusted valves, and that I'd be buying a car that I could fix cheaply.

I've done the valves and that got me more power, but no change in the nailing sound. I also changed filters all around.

That led me to research and this board. There is a lot of information in this board on "Nailing" generally describing it like the sound of a hammer nailing.
hammer nailing nails

In trying to find the problem, I then cracked open each of the injectors in turn, and that hardly or did not change the nailing sound, each time engine speed dropped, and idle became rough, but almost no change in the nailing sound.

I borrowed a mechanic's stethoscope, and found that the nailing sound seems to come from low down near the front of the block, right near the oil pan and the pulley the fan belt rides on.

I also notice that from underneath, the fan belt to some extent is wobblying in time with the nailing sound.

I'd like to hear some opinions on what to do next, and Is this likely to be the type of problem that destroys the engine if I keep driving it as is?