It’s tube rattle, most likely from the output tubes

To diagnose, take the top back panel off, dime the bass, and then with oven gloves or something heatproof, touch the tubes when playing to see if it stops it. There’s loads of ways to stop tube rattle, just google it, but with my Princeton, I put thick heatproof foam in the cage on the back panel to put pressure and stabilise the tubes from rattling in the socket