I had my passenger-side rear tire up in the air to check my work on the fuel filter. (Bone dry. It must have been the rusty car next to me reeking of gas.) I decided to spin my tire, and about half the time it was spinning, it made a faint humming sound.

I know a grinding or whirring sound from a wheel bearing means it'* time to get a new one, but what about an intermittent hum? Are the H-body'* wheel bearings supposed to be completely silent?

Keep in mind that the pads are typically contacting the drum when not engaged. Did you get a vid of said noise?

If the pads are touching the drum, then that would explain it, and explain why it only happens half the time.

I thought about taking a video, but there was a chirping bird that was drowning it out. (I said it was quiet.)

If you push and pull on the wheel when grabbing at top and bottom, repeat when grabbing at the sides, and there is no play, the bearing is likely okay. By the time they start making noise there is generally some play.

Zero play in the wheel bearings. At almost 220,000 miles, I'm pleasantly surprised.

When a wheel bearing is going bad it often makes a droning noise similar to a light aircraft flying nearby at highway speeds.

If you aren't getting symptoms like that and the bearing has no play I suggest if it ain't broke don't fix it.

Roger.