(This is what my head is like. Your head may vary.)
There's a set of tunes really close to the surface. If I'm walking along, absently, not really thinking, there's a high chance I'll be whistling, humming or singing one of a limited set of tunes.
I started trying to catalogue them a while ago but it's quite hard to do, I'm really not conscious of them, noticing them is like noticing your breathing. But I've realised that I've probably spent more of my life whistling the guitar riff from 'On The Beach' than performing any other non-autonomic function.
Nevertheless I've made a start; there's an audio file above.
There's another set just a bit further down which gets triggered by the world. So if someone says something like 'give me a minute' my head instantly sings 'Give Me Just A Little More Time'.
It works with words too. If someone says 'mild', I think 'mild hairy lip squid'.
(UPDATE: @martinpolley points out that it should be 'mild *green* hairy lip squid' but that seems to have dropped from my memory/reflexes)
That's the next list.
Anyway.