Why does my tube amplifier make noises after being switched on?

The tubes expand as they warm up. Inside the tubes, there are different materials such as glass, nickel, copper and other metals, which expand differently from room temperature up to around 700 degrees Celsius. This can cause noises in the tubes themselves, but also in the speakers, and is usually harmless. After the break-in phase, this will also calm down somewhat, as all materials find their position. For this reason, high-quality devices often also have a switch-on delay.