It is convenient to classify rhythm as unpitched sound but rhythm is an integral feature of pitched sound as well. The parts of this guide titled writing melody and writing harmony should really be titled writing melody-cum-rhythm and writing harmony-cum-rhythm. This would emphasise the point that rhythm in melody and harmony is built into the notes, it is not a separate feature.
This section provides a bridge between rhythm and melody.
First it reviews pitched percussion instruments.
Then it looks at the two main ways a note differs from a beat: