Engine noise

i have nissan pathfinder and have done the engine overhaul 1 week back after the overhaul i took the car and drove around 600kms and seems to have a noise at low or on idle rpms of tappets or lifter and doesnot go away from the day one. i have asked the mechanic about it and said it will go and but it is increasing. sugguested an oil change. any idea what might be the sound from.

It sounds as if you are hearing valve noise. The good news is that your block is not showing signs of failure after the overhaul, but I wonder if the heads were rebuilt or about the quality of the work.
An oil change is necessary within a thousand miles of an engine overhaul. Do it anyway. But you may still hear a tapping noise from the upper engine that might need work. Do the oil change and then see what happens. If there is still a tapping noise, you have a sticking valve or a bent rod in a head and that needs to be repaired.

thank you very much i really appretiate your reply today i have taken car to the same mechanic again to check the engine noise with the help of esthtocope he checked and took to another mechanic to have his findings right both said its from the timing chain and has to be replaced to rectify the sound. one more thing is that sound only comes when engine running in idle and have the operating temperature. on cold start i don’t hear any sound. the heads were normal and no facing was done only surface polish with new head gaskets replaced. i will change the oil after 400 kms.