The high notes returned to his compositions towards the end of his life, which suggests he was hearing the works that were taking shape in his imagination.
I'm not sure how to write up the syntax tree for this sentence. To be specific, what is the position for which-clause in the tree. I think it is an adjunct of the whole clause [The high notes returned to his compositions towards the end of his life] so it may be put as an adjunct of TP but I'm not sure how to write it.