SPOILER ALERT: tl;dr

I'd suspect that the sounds you're hearing when nodding off are called "hypnagogic hallucinations." They're p common, and they take all kinds of forms. Almost everybody gets one at least once in their lives, and folks with bipolar and anxiety issues are especially prone.

Sometimes people experience them as a heavy, even suffocating, weight on their chests. Check out this cool Fuseli painting:

https://artsandculture.google.com/as...gA?hl=en&avm=2

The hallucinations themselves are harmless, but they can be a sign of narcolepsy. If you have the insurance for it, might not hurt to get a sleep study done.

As to your first question about martial arts, I strongly recommend it. Tai Chi has helped keep my head straight in the decades since my BP2 started manifesting. I do monitor my moods, and I back off from the contact stuff when I'm feeling flaky, but there's always newbies to teach, floors to sweep, etc. You're always learning when you're willing to let yourself.

Good Luck