My boat has a Volvo MD32A naturally aspirated 6 cylinder engine with 1500 hrs. according to the hour meter. Engine was last produced in 1977. A valve job was done by others in 2008. I bought the boat in May 2009.

After about 10 hours of use, the engine began making a hammering noise - first thing I thought of was bearings, but the noise went away within minutes - came back again - went away again, and on and on.

About here I noticed the coolant level in overflow bottle was diminishing - cap was leaking - installed new cap - no more leaking there. Coolant level still dimishing, but slower.

People offering advice said probably water in fuel - run it hard.

After a few filter changes, I removed the fuel tanks and cleaned them (they were nasty) - I could get my arm in far enough to clean the entire inside - new filters again and fresh fuel. Trial run - after about an hour, noise started again - went away - started again. Every time the noise begins, I change throttle setting, either more or less and the noise will go away - for a short time.

Any thoughts here ? Could it be injectors ? Head gasket ? Maybe the others who did valve job didn't re-torque the head after 20 hours as directed in manual ?

Other than this noise, the engine runs smooth as a silk.

Thanks in advance