It could be an earwax buildup (if it leans against your eardrum, you can hear yourself brushing your teeth very nicely) or just a swelling of the tissues around the ear canal limiting the ability of water to get out after a shower or wax to drain out. After all, pregnancy makes our nasal passages swell, why should the ear canal get a pass?
I kind of get the same thing, its like my ears plugged/has water in it. I sway my head and there's a woosh sound. Like you, I can hear my heart beat and wind/my breathing. I'm just as lost as you are :T
Btw also 1st pregnancy, 23 weeks +1.. With possibly a girl...