Hydraulic lifters are part of the valve train. They use oil pressure to compensate for wear and maintain consistent valve clearance. When you change the oil, especially if you don't carefully prime the engine or if the oil is slow to circulate, the lifters can temporarily lack sufficient oil pressure. This causes them to tick until they are properly lubricated.
The ticking usually goes away after a few minutes of running the engine. If it persists, there could be a more serious underlying issue, such as low oil pressure, a clogged oil filter, or a problem with the lifters themselves.
