Piston slap doesn't sound anything like that. It's almost certainly something at the top of the engine that's knocking until there's enough oil pressure to silence it. Perhaps a bad lash adjuster or cam phaser. Could also be a clogged VCT solenoid.
2006 GT Deluxe - 5MT - 12.50 @ 109.61
314rwhp 330rwtq with basic bolt-ons