For me, every phase is different.
Normal modus operantus:
  • I hear (or read) someone say something that catches my interest.
    Most of the time it's just a short phrase that my brain immediately transports into a completely different context.
    It's hardwired, I can't really stop it.
    The idea for a song is born.
  • I noodle around on my acoustic until something useful falls out of it, and bring those chords into BIAB
  • From there, melodies and harmonies develop in my head (or, as you said, I 'hear' them).
  • The lyrics develop when I walk the dog .... I am a dog-walking-writer smile