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New as in Forged aluminum low silicone alloy?
I've heard those pistons make a similar sort of noise. yes, 'piston slap' because the pistons need to warm up when they are set to manufacturer recommended piston to wall clearance.
But,
1. I've never heard it that loud before
2. If it really is cold piston slap, that noise is the pistons gradually wearing the bore into an oval shape (it will have scuff marks down the lateral sides of each bore)
3. Engines built with forged aluminum pistons and massive piston-wall clearances are not intended for cold start or to be operated while the oil is cold. They require oil temp minimum of around 170-185*F Before you can load the engine, otherwise, loading the loose piston from the top will create additional force which will further accelerate the bore wear.
generally, forged are not good pistons for daily drivers or street cars unless a low expansion aluminum specific alloy and closer piston-to-wall is used.
Now all of that said, if you told me those pistons are new but they are cast aluminum, I would suggest you have the wrong size pistons, improper machine work, something like that. It shouldn't rattle with a cast piston.
Also when you make a thread asking what this noise is it would be very helpful if you had mentioned that all of the bottom end parts are new, or the engine was just rebuilt, etc... Because in the video I and from the post I assumed it was a 200,000 miles high mileage beat up ruined sr20 engine and there is no way to tell any difference.
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