ABRASIVE FIBRIN FILAMENTS & THE PERICARDIAL FRICTION RUB In acute pericarditis, the once-smooth pericardial layers become inflamed and roughened by abrasive fibrin filaments. Instead of gliding effortlessly with each heartbeat, these irritated surfaces scrape against each other—creating the classic pericardial friction rub. This harsh, leathery, sandpaper-like sound is often best heard at the left lower sternal border, leaning forward, during end-expiration. It’s a small sound with big clinical