what's very fast ? (frequency in 1/s)
where is it ? (chest ?)
I'd try to attach a stethoscope-recorder and replay it to the doctor
(.. and to others here)

According to my NEURO, the MRI is to look for any abnormalities of the spinal area that could relate to a nerve or nerve pathway setting off these vibrations (i.e., it could be indicative of a neurological disease or not a disease but some other acute or chronic condition related to inflammation, injury, etc.). For myself, I'm thinking it's going to be negative (this is not me sleeping in an odd way, pressing a nerve, etc.) since it's aligned to those 10-20 seconds of coming out of sleep. My NEURO is smart (I personally have a PhD in NEURO and feel a good judge of that) but he's never heard of internal vibrations and has no patients with them (and he originally confused our conversation with "tremors" related Parkinson's or MS, which is not surprising). But we'll see....my MRI is Friday.