I often find myself singing an ever changing melody while I am on long distance drives.
My wife and I love to drive. Our poor vehicles have logged thousands of miles as we've traveled back and forth from California, Colorado, Wyoming, Washington, Utah, Nevada, Oregon, Oklahoma, Texas, Kansas, New Mexico and the pretty much the entire isle of UK from John O'Groats to Plymouth. While driving we chat about pretty much everything. Occasionally, the day gets warm or the night gets long and my wife drifts off for a brief nap.
Unfortunately, there's no way to write any of these improvisations down as I keep my eyes on the road.
It is during these 'quiet' moments that I sing to myself. Sometimes it's jazz influenced, other's are far more classical in nature, even venturing into Baroque stylings. Most of it is pretty useless, variations droning on with no real direction. However, every now and then, I glob onto something I really enjoy, a melodic line that intrigues me in how it fits together. Or a unique rhythm that takes me a while to sort out just how it's put together.
Yesterday, on the road from Denver to Fort Morgan I wound up with a nice rhythmic line of 5+5+6 which then lead to a series of 6's or (3+2,3+2,2+2+2 repeated twice then (2+2+2+3+3+2+2+2). The shifting sense rhythm was cool to the point I ended up getting my phone recorder out to record a snatch of it to save the rhythm for later.
There were a couple more nice sections during the singing, improv session in the car yesterday, but none of those were recorded. So, they're gone, lost. Maybe some day I'll sing something like it on other drives, but if not... ideas are a dime a dozen! It would be nice to get all these ideas down, but I've come to realize it's not the idea that make great music (although it helps to start with a good idea). The pieces of my music I really enjoy are the ones that were not just good ideas, but concepts that were crafted into a fully formed piece.
So, for all those budding (and not so budding) composers out there... don't worry so much about finding the "right" idea. Start with anything and spend time crafting it. A rock is just a rock until you unlock the statue inside. Music is that way. Chisel away at the rough stuff. A David is waiting to be discovered.