Have you had the plugs out, are they the right ones, have you ran the engine in the dark, any cracks on the spark plug body will show up, also a quick test for the followers(tappets) is to take the rocker cover off, turn the cam with lobes up one cylinder at a time and get a slim flat screwdriver between the back of the cam and follower, twist the driver a bit and feel for a bit of slack, there should'nt be any slack it should feel nice and smooth and you should see the follower going up and down. check the bottom cover by the alt belt, its gets really close to the grommit/plug for the alt bolt, it really doesn't sound like a mechanical failure, more like a something coming around on a loop, if you count the beats of the tapping its more like tenth engine rpm as the crank/piston is going to be doing 6 taps a secound at 1500 rpm and its not that many, even the tappets/sparks will be 3 times per second as they are half engine speed, thus my thinking on something on the loop of a belt etc....