My 2009 Jetta TDI makes an occasional loud engine ticking sound when started and for the first mile or so when driven. The car has 15500 miles. It didn't make this sound before reaching the 14,000 mile mark. Oil level is fine although was over filled by dealer at 12 month and 24 month service which I had corrected both times. Engine runs smooth. Any idea what could be making this sound? Should I be concerned?