When the engine is initially started up, it takes a few seconds for oil to get to the top of the engine. As the oil starts flowing and the engine warms up, the noise will generally go away.
Using the correct oil for the vehicle will usually reduced the length of time it takes for the oil to reach the top of the engine. Higher quality oils will leave more of a film on the engine to prevent "dry" start ups.
On the back of the bottle of oil, it will say "approved by". Look for oil that says "approved by Toyota" on the back of the bottle. Oil approved by Toyota will contain required additives to prevent dry startup and internal engine damage.
Some engines use hydraulic lifters that operate off oil pressure. If a lifter is starting to fail, it may also result in valve train noise.
〉 Answered on Jan 15th, 2016 by Helaine Kurot, Owner/Technician at 360 Automotive