Just a thought, has your engine got good clean oil of the right grade and up to the correct level, possibly this noise is dut to an hydraulic lifter blockage, they have very small oil passages that can clog if the oil bacomes dirty.
If you put the handle of a screwdriver against your ear and touch the buisness end on various parts of your engine in the general area of the noise, you should be able to narrow down where the noise is coming from, this may give some clues, please be careful to avoid rotating parts with hair clothing etc and spark plugs/wires with metal objects, we dont want to add any more noises
Good luck
James
