1. Earbuds muffle external sounds, so you'll more likely notice internal sounds, like swallowing or your heartbeat: 2. Earbuds fill your ear canal, and as your

1. Earbuds muffle external sounds, so you'll more likely notice internal sounds, like swallowing or your heartbeat: 2. Earbuds fill your ear canal, and as your ear canal flexes slightly due to your heartbeat, the earbud rubs against the skin inside your ear, and you hear that.