Does my MacBook Pro have a fan? I thought all machines were solid-state.

My previous Mac was an 11 inch MacBook air, and I remember being told when I bought it it was amongst the last Macs to have a fan, as all subsequent machines were solid state and didn't need them.

I bought a MacBook Pro 15 inch eight months ago and occasionally, for example when installing a new operating system, there is a fan sound – or what sounds like a fan. Is it? Or is there something wrong with my machine?