Looking for Quiet External HD...

You could try buying you own enclosure and drive of choice. Also, SATA drives have an Acoustic Management option that is usually "performance" mode as shipped, not the quietest that drive can do. I don't know if Windoze (or MAC) gives you the ability to change it or not with "as shipped" software. I've never tried it except on Linux (hdparm) or in a device driver.

If you have any audible noise, especially in an enclosure, from an idle drive, something could be wrong. An idle drive, just spinning, will have some noise but nothing an enclosure won't muffle. Some enclosures have fans which create noise and don't have long life spans.

The noise you hear when I/O is happening is from the heads/actuator moving. This is what the Acoustic Management feature controls - how fast the arm moves, etc. It's a safe setting to change. Some drives that are marketed for DVR use may already have the Acoustic Management set to "quiet".

And, if your drive is mainly for playing back music files, you should not really hear any noise from the drive because most, if not all, of the I/O should be sequential. Keeping drives de-fragged helps.