The rub might not cause pain because it isn't rubbing on a part of your pleura that's hideously packed with pain receptors. A good thing to know is that your visceral pleura (the stuff wrapping your lungs) has no pain receptors, whereas your parietal pleura (or the stuff that lines your ribcage,) does. So if your parietal pleura isn't getting rubbed on, or the rub isn't very big, it probably won't cause pain.
Yes, a rub might stay for-ever-and-ever. Some things might even CAUSE a rub. I have a friend who had a rather nasty pleural effusion after surgery for a burst appendix, and had to have a chest tube inserted. She now has a very distinct pleural friction rub over her right lower lobe, posteriorly. It's just scarring that's happened on the pleura that's touching. It doesn't cause her any pain, but it sounds MOST interesting with a stethoscope. Sounds like crackles, but you can tell they're not because it makes the exact same pattern of creaking no matter how she inhales, and exhales. Creak-creakcreak, creak. Creak. Creakcreak.
The thinner you are the less bone and fat and tissue that can distort the sound. Kind of like listening to someone speak through a thin wall as opposed to a thick wall. Ne?
Maybe the rub didn't match to your heart rate, like a pericardial rub would? I don't know. But like I said before -- rubs are deceptive -- they can sound like a lot of things.
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