During the day your mind stays busy with tasks, which keeps thoughts running in the background. Once you lie down and outside input drops off, your mind starts processing everything it postponed during the day, often all at once.
That's why even after a genuinely calm day, bed can bring an unexpected rush of thoughts. It's not a malfunction, it's the mind catching up on work it couldn't do earlier.
A light, steady sound helps here, because it gives the mind something neutral to settle on. The thoughts still show up, they just stop piling on top of each other so fast.