There are two different sounds possilbe--one is caused by the piston top hitting on deposits in the combustion chanber--running a can of TECHRON in the fuel tank will remove lots of the deposits.

Other cause is 'nailing' which is caused by poor spray pattern on the fuel injector, very poor fuel quality (low cetane number), low compression, or injection timing too advanced. 'Nailing' is what happend when a lot of fuel gets injected before any of it starts burning. Defective glow plugs can cause it too. Anything that interfers with the injector putting out a fine fog of fuel will cause the nailing. There is a good injector cleaner on this web site (1pint can) that will usually clean up mildly fouled injectors.