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If oil pressure is in question then the first thing I would recommend is checking the pressure with a manual gauge, starting from a code start. The pressure gauge on the dash isn't usually that close of a representation of actual pressure, and there are always potential electrical problems that can affect the gauge.
If pressure is actually low when the noise is happening then you have a bad oil pump, restricted pickup screen, or possibly a drainback issue due to sludge buildup in the engine.
It almost sounds as if you're describing piston slap, which to a degree is normal. Piston slap happens when the pistons are a little small for the bores and they make noise as the skirts hit the cylinder walls when they change direction at the top of bottom of the cylinder. As the aluminum pistons heat up they will start to expand and the noise will start to subside. Piston slap isn't uncommon and can be considered normal to a degree, but if yours has suddenly gotten worse then you definitely have a problem.