I've a 350.5 with the same popping, clicking on powering up and off. I spoke to a Pass Labs tech, he explained it's entirely normal. After shipping, the unit may have sustained some hard landings. To mitigate by perhaps 50% these sounds, he suggested after the amp has been powered on for a couple hours, to take an allen wrench, loosen and remove the front plate, and tighten (but not too much) the chassis screws inside. I haven't bothered with the fix; knowing that this is normal operation is OK w/ me.
