The first has to do with priming. My Silvia manual suggested I run some hot water through the steam wand every time I turn it on. The Alexia doesn't say anything about doing this.

When I turn the Alexia on, I can hear it beginning to boil almost immediately, and then often there is a loud thunk sound. This didn't happen when I first got the machine. If I turn it on and run some water through the group, this doesn't happen. I'm just wondering if this is normal. I really don't want to burn out the boiler.

Second has to do with descaling. I purchased the machine through Seattle Coffee Gear, which in turn shipped the machine from ChrisCoffee. ChrisCoffee recommends against descaling, and I'm using water that tests less than 70 ppm with an electric TDS tester. Seattle Coffee gear recommends that I descale, but that I shouldn't run the descaler through the steam wand. Other places say that you should descale, but that you shouldn't run the descaler through the grouphead.

There's a lot of contending information, so I guess I'm just wondering what people have done that has been successful.

I believe the sound you are hearing is the water boiling at near room temperatures. This happens on machines without vacuum release valves. Alexia doesn't have one, right? There is never air in the boiler, just steam and water or just water depending on the setting you're on. When you turn it off, the steam settles back to water and leaves a void in the boiler, a vacuum. Water boils at a much lower temperature in a near vacuum like this. It sounds out of control as well. Whether it is harmful or harmless to the machine, I don't know. If you open the steam wand though, it should stop and continue heating. This is my guess but I'm almost positive that this is what is happening.

This sounds pretty feasible. I can't find anywhere that it says anything about a release valve, but what you said would explain why it calms down when I run some water through the grouphead.

However, I'm not really concerned about the boiling water sound. It's the loud pop or thunk sound that happens about 30 seconds after I turn it on. It sounds like someone hit the casing with a large marble or something.

It could just be the metal heating up, and I assume it's probably nothing to worry about? I have no idea

My machine (before I put a vacuum breaker in) made the same sound. Sounds like a little explosion. It actually would make me jump. After a few times I knew I needed to equalize the pressure. I still have no idea why the sound is so intense but at least I was able to figure out exactly what was happening. My machine is an HX, so yes, for your single boiler, opening the steam valve or opening the group valve should equalize the pressure. As I think about it, I don't think I know of any single boiler machines with a vacuum breaker. Perhaps it is an element that cannot be applied to a brew boiler and/or dual use boiler.