obviously not at all an expert or even qualified in the fields of neuroscience or psychology, so speaking as a layman, but I think it's all a continuum, from your external sense organs like ears and eyes via the connecting nerves to the inner reaches of the brain.

I think the initial stimulus can originate at any part of this circuit.

So I do not think there is much difference between external so-called real stimuli and internal ones.

it's all just vibrations, processing, and reverberations :)