If any of the above situations occur, extraneous noise can be eliminated by removing air from the hydraulic lifters.
Check the condition of the engine oil and top up or replace it if necessary.
If the oil level is insufficient, air will be sucked in through the oil intake with a mesh and will enter the oil channel.
If the oil level is higher than required, the oil will be mixed with air by the crankshaft and there will be a lot of air in the oil.
If the oil is old, the air will not separate from the oil quickly and the proportion of air in the oil will increase.
If air with oil gets into the above-plunger cavity of the hydraulic tappets, the air inside the above-plunger cavity will be compressed when the valve opens, and the hydraulic tappet will not be compressed enough, which will lead to the appearance of extraneous noise when the valve closes.
If you remove air from the oil, the operation of the hydraulic lifters will return to normal.
Let the engine idle for 1-3 minutes to warm up.
Increase the engine speed to 3000 rpm for 15 seconds, then reduce the speed to idle and let the engine idle for 15 seconds, then repeat the cycle and check if the extraneous noise disappears. During normal operation, the sound disappears after 10-30 repetitions, but if after 30 repetitions or more the sound level has not changed, the probable cause of the extraneous noise is not the appearance of air inside the hydraulic lifters.
After the extraneous sound disappears, repeat the cycle 5 more times.
Let the engine idle for 1-3 minutes and check if the sound goes away.