Could be a PVC/Oil Trap/Flame trap issue. With the engine running and your oil cap off, is air being forced out of the oil port? Is there air coming out of the dipstick port?

The timing on most new cars (including many American cars) is controlled by the cars computer.

Check and make sure your exhaust manifold bolts are tight, I have seen some come a little loose causing a valve tap like noise.

Are you using quality oil and changing your oil on time? Try an oil change ...