Tapping noises from the engine compartment of a minivan lasting more than a minute but dissipating once the engine is warm are normally from leaks in the exhaust manifold. A tapping noise that lasts just a few seconds on startup is usually due to malfunctioning valve lifters, according to Auto Repair Help.Continue Reading
A visual examination of the exhaust manifold helps to determine if this is the source of the noise. Signs of leaks include black soot marks where the manifold joins the cylinder head, and cracks in the manifold or missing bolts are another common cause of this noise. Many small leaks seal upon heating and expansion of the exhaust system. Repair involves replacing the defective part.
Valve lifters use pressure from the engine oil to expand the lifters, providing a zero valve lash clearance, and preventing the tapping noise. A spring and valve keep the lifter expanded and filled with oil when the engine is not operating. Failure of either of these components causes the oil to drain from the lifter, resulting in collapse. It takes a few minutes for the lifter to build pressure to operate without tapping.
Repairing oil lifters involves a major engine disassembly, although oil additives are available that sometimes free the sticky lifter valve.Learn more about Engine