An engine ticks after an oil pan replacement because of a malfunctioning lifter. Another cause could be a bent push rod. Both problems warrant a trip to a professional mechanic for further diagnostic evaluation.
Ticking in the engine is called a lifter tick. It may be caused by dirty oil, in which case the replacement of the oil pan eliminates the sound. If the sound remains after replacement, it is because the camshaft and pushrod do not make contact with the lifters. This happens because of the loss of internal oil pressure in the hydraulic lifters.
Malfunctioning lifters in the engine require removal and reboring. The process could require only an adjustment of the engine's valve covers, or the damage could be extensive enough that all of the lifters need replaced. Before running to the garage, owners can attempt to remedy the solution by using an additive like Tappet Stop Noise, which lubricates hydraulic lifters. It also cleans the engine's bores and valves and has no impact on oil viscosity.
Bent push rods are the most expensive cause of ticks in the engine. It is caused by overly accelerating the vehicle, which increases the depression force on the rod. Bent push rods cannot be repaired; they must be replaced.