Kirr Kirr like sound is usually produced by relays, but the sound should come from fuse box. If this is the case, the fuel pump relay/connector might be faulty.
By your statement it looks like engine knock sound, that by the way sounds more like "Karr Karr". If it is so, either get an advancer installed or mark two point on distributor after observing the ideal timing for petrol and cng (you can get help of some mechanic). While driving on petrol, turn the distributor to petrol mark and on cng turn the distributor to cng mark.