That sounds like detonation (pinging) usually caused by excessive timing advance, poor fuel quality, to high heat range plugs, etc etc... If it is pre-detonation, you are burning holes in the top of your pistons. Motor wont last long. Keep trying to find someone who recognizes there is a problem. Sometimes it's hard to find a good mechanic.
Good Luck!!!
__________________ “If it rolls, floats, flies or shoots, runs on gasoline or gunpowder, goes fast or shoots a big bullet and makes lots of noise, thus producing torque and recoil — it’s cool.”
--Sterling Moss, Society of Torque and Recoil
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Please Help, diesel noise 
