I remember my mum telling me how my dad could never understand what I was talking about (aged up to 8 - didn't see so much of my dad after that). I used to make up words, but particularly saying words backwards and "spoonerisms" (swopping first bits of words), e.g. spontaneously thinking aloud how we were in the car approaching the "krap rac"(car park). Just today ( I found myself smiling as I picked up my "bopping shag" (shopping bag) (shag = slang for having sex, for any non-native English speakers) though I didn't spontaneously say it aloud.
I think a recent example of idiosyncratic humour is, upon reading "S & M Pickering" I thought it sounded like a kinky marriage proposal "S&M? Pick a ring!" This is a good example of what goes on in my head, sometimes finding myself amused, but unable to say why - others would probably not appreciate it (going by their previous reactions) especially if they're trying to say something serious..and yet life would be so much duller without my inner entertainment. If I am with someone who knows me well personally, I can usually share the joke. It's the more formal, superficial relationships which are harder and I do my best to suppress my thoughts and reactions.