If an agent uses another entity in order to perform an action, that other entity fills the role of instrument. In the sentences The boy cut the rope with an old razor and He drew the picture with a crayon, the noun phrases an old razor and a crayon are being used in the semantic role of instrument.

When a noun phrase is used to designate an entity as the person who has a feeling, perception or state, it fills the semantic role of experiencer. If we see, know or enjoy something, we’re not really performing an action (hence we are not agents). We are in the role of experiencer. In the sentence The boy feels sad, the experiencer (The boy) is the only semantic role. In the question, Did you hear that noise?, the experiencer is you and the theme is that noise.