UnderdogA little "rattling" or "tapping" on a cold start after an oil change is totally normal. How your ear perceives that is highly variable, I'm sure. And there are lots of things that can be making noise due to lack of oil pressure on a modern engine. There are the traditional noises like bearings on the crankshaft (not likely, as there's still plenty of oil there), and "piston slap" from the not-warmed up piston scraping against the not-quite-lubricated cylinder wall. And we have a timing chain, so there's potential for noise there with no oil pressure. And we have an oil-operated variable valve timing control system that could potentially make some noise while oil pressure comes up. It takes oil pressure 3-5 seconds to build up.
Protection from this kind of thing is what synthetic oil is good for. You have a good film of synthetic oil on all of the moving parts even after draining the oil. Something might have enough clearance to rattle a little bit, but it's not going to hurt anything. A 900 rpm idle speed is 15 revolutions per second. Your four seconds adds up to 60 revolutions of your engine, probably less.
There are also other things that make noises under the hood that you might not notice if you're not thinking about them. The ABS pump does a self-test/purge cycle at some point after startup. Fuel system purge control solenoid. It's hard to say for sure what you're hearing. But if it's only happening for 4 seconds at startup, and only immediately after an oil change... it's not a problem.
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