What causes noise when a plane is on the ground? What's the loud continuous noise the plane is making when it's on the ground and the (jet) engines are visibly not running (spinning, at least) ?

I was boarding and I thought it was perhaps a compressor for the cabin air but when I entered the cabin it wasn't cool inside.
That would be an Auxiliary Power Unit, a small jet engine that is used to provide power on the ground when the engines are off and when there is an emergency in the air.