And its not in the metal way.
He does it in his cot whilst on all fours and rocks back and forth bashing the top of his head on the cot. He also rocks back and forth quite hard whilst in his high chair which hes done since being 6m old.
He's not a tooth in his head and his ears are clear as I had them checked a month or so ago.
The cot shenanigans have been going on a few months too and sometimes its during the day and sometimes the night.
Is it just normal behaviour? DS1 was never an headbasher.