You can always take a small hose, like heater hose, and put a piece of paper towel in one end (to diffuse wind) and then use that end to place against the engine, place the other end next to/into your ear and set the end with the paper towel against various places on the block.
This could help you ascertain where the noise is coming from.
Check the valve covers, injector bodies and the bottom of the block. Depending on where the noise is loudest that could help to narrow down your potentials.
The hose should be a 2-3' feet long, it's kinda like a makeshift automotive stethoscope:thumb:
Does the noise your hearing change with RPM? Get louder, or quieter, stay the same or increase repetition with RPM?