Yo guys i need help with my car. When i start my car's engine the time it is cool, there a strange sound appears like " trrr trrr trrr" u can say vibration when some metal strikes with metal something like that. Once this voice came as well before and i got it checked up from HONDA 2S PIT STOP BAHAWALPUR my mechanic changed the position of CNG pipe at that time it was fixed but again the voice seemed to appear after one week of that day, i re-adjusted that pipe again but there is no change that sound makes me really pissed off please help what could it be .... ???