I had a similar problem once after I had done an oil change, was fine before then after 100 miles a tapping noise started - I swapped the tappets and still no joy.

In the end I pulled the motor apart and it had knocked out the crank bearing closest to the flywheel - right down to the metal and had then spun.

The sound is very similar to a rattly tappet - my guess is the oil change loosened a particle in the motor which blocked the oilway to the bearing.

Everyone that heard the sound said "rattly tappet"