From personal experience, intrusive thoughts are usually the manifestations of mental instability. It's the worst parts of yourself gaining power. Are you under heavy mental stress? Back when I was about 11-12 mine got so bad they gained sentience. They don't really go away, but you can learn to ignore them. If you want to, check for vitamin imbalances you might have. That could help. It could even be completely psychosomatic, and they only exist because you think they do. Deny their existence and they might leave.