My spectra used to do the same, knocking is because of advanced timing on CNG and its not the same as Petrol. u need a good CNG adjustment. thats all. i could have helped u a lot if u were is Islamabad, my cousin is in Karachi, his workshop is in Zamzama. if u want to contact, his name is Muzammil and his # is 03332170727, just tell him, Saqib from Islamabad gave u this #, he will surely b of help.