Hi mummy 😊 yes quite a lot of babies around that age do this, my own also went through that phase 😅 Around 5–8 months they start discovering their body movements, so shaking head side to side can just be them playing, soothing themselves, or reacting to sounds/attention. But if you notice it’s very frequent with other concerns like not responding to sounds or seems “stuck” in repetitive movements, then maybe just mention to PD for peace of mind.