We had a similar post some time back, and most of the regulars concluded that when the valve lifters on type of engine wear, the oil drains from the valve gear, and upon startup, they are noisy until the oil pressure builds up, when it disappears. Nothing to worry about; my Chev Impala with th 305 V8 did this for several years. Don’t put heavy oil in your engine, as some might be tempted to do. That will create many other problems, including rapid engine wear at startup. You might also notice a small puff of smoke from the exhaust after startup; that is some oil draining past worn valve guides into the cylinders, resulting in some oil burning. This is also nothing to worry about unless it is severe. If you need to go for a smog test, make sure the engine is well warmed up, since the burning oil will make you fail the test.