

I’ve heard that a lot, and I always thought it had to do with lazy talking (not moving your mouth much). I do think that’s part of it, and some people do that more than others.
But the thing about having a potato in your mouth is it pushes your tongue down and back, into your throat. Which is something you need to say the American “r” and a lot of other sounds, like “w” and the dark L like in “pull.” It’s hard to teach, and very hard to unlearn. It’s part of the characteristic American sound.
German is basically the opposite. You can see the difference here, with a German speaker talking in an MRI. There’s a lot of space between the back of the tongue and the back wall of the throat. And here’s a picture of someone saying an American “r”. The base of the tongue is all bunched up in the throat.
I’ve been able to hear that too, for as long as I can remember. I don’t think that’s necessarily anything unusual when you’re the type of person who can hear earthworms cough. It sounds different at different times, like if I’m stressed or dehydrated.
There’s really a lot you can hear if it’s quiet and you pay attention. Eyes opening and closing, eyeballs moving, joints moving (neck and spine sounds different than knees and elbows), muscles contracting (different ones sound different).