Android phone makes random noises when silent

[AC Question]

English isn't my first language and I'm sorry for grammar mistakes. For the past few months I've had this issue with my Android phone. It happens only when I'm not using it, and sometimes it also makes random notification sounds. The sounds I hear when I'm not using it are clicking noises and a static sound. Once I woke up in the middle of the night because of them. I never download anything, and I only have a few games installed on it which I've never had any issues with. I've had these games for 2 years now. I don't think they cause the sound because they don't give me notifications. Nothing shows up in the status bar either, and I don't really call anyone because I only have a few friends at college. My phone isn't connected to Bluetooth, Facebook, and twitter. My phone is 5 years old. What could cause these noises? Thanks in advance!