Just tried a few times so as to figure out a way to to describe as best as possible how to make at will that inner ear "flutter" sound, those mild low frequencies.
The best way to describe how to make it is to somehow replicate what you start to do when you yawn. You don't need to yawn or move anything in your head or jaw actually, but the way I make that sound is to think as if I was right about to start yawning. I just don't start yawning, yet that flutter can be perfectly heard after that minor tension somewhere inside my head, similar to something involved at the beginning of yawning. Once you catch it, it's pretty easy to replicate. The sound is just a minor flutter, as if there was a very soft wind blowing right in front of your ear drums. A kind of brrrrrr... from a low level subwoofer, but coming from right inside your ears.
One interesting thing, I can restart the flutter at any time, but I can't sustain it for too long, just a few seconds.
PS. That is a description of how to achieve a perceptual experience, and a description of that experience. Kind of describing how to snatch those 3D images from a stereogram. Somehow describing how to achieve a particular perceptual experience makes me think I'm writing like Patrick (no offense Patrick
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PS2. Truly after a few times, I'm realizing the "tension" causing the flutter can be felt precisely inside your ears, not in the muscles on your head around your earlobes. Well, not sure whether bone conduction would cause a similar result misplacing the apparent location of the tension.