Mine's been doing this for six years so i figure it's nothing to worry about.
I've never heard a definitive answer, so here's my theory:
When the engine's cold and the oil is thicker, they tend to muffle the noise of internal engine components.
As the engine and oil get hotter, they're better conductors of sound, plus the oil is thinner. I'm pretty sure what I'm hearing on mine is the whirring of the cam chain.
I'm not an Acoustical Engineer, but I did stay at a Holiday Inn Express last night.....