I’d be frustrated too, especially with a brand-new Epiphone developing the same rattle twice. A loose pickup or mounting hardware could definitely cause an intermittent vibration, but I’d have the shop inspect it rather than trying to open the guitar yourself. P-90s have several components that can potentially vibrate or resonate. Since the guitar still sounds great, hopefully it’s a simple mechanical issue rather than anything serious. I’ve also been checking out Crowngreen recently, and it’s interesting to see how different communities share advice about their hobbies and experiences.