To help pin-point the source of the noise, use a long screwdriver and place the metal end on the engine and place your ear to the handle of the screwdriver.
When you get to the area which is making the noise it gets transmitted through the driver and you'll hear it loud and clear.
That's an old trick my mechanic (sadly now deceased - which shows how long I've had the car!) taught me.
Move the screwdriver to various parts of the engine to find the noise. It's the best method for finding the exact tappet that's making some noise, and also for pinpointing bad bearings.
Of course, be safe and keep the driver away from moving objects or you may have to go to the hospital to get a screwdriver extracted from your ear!