I just replaced mine this spring with a recent model Frigidaire I got almost new off CL.  The fans runs all the time. 

But once in while (every hour or so) it does shut off, like for a minute or two, and then turns back on.  No idea why it does this.  It's quieter overall than the 15 year-old one it replaced.

9 years isn't a bad lifespan for one, I read a Consumer Reports article that said many of the newer ones aren't lasting more than 3 or 4 years.

If it is electronically controlled, perhaps there is a relay on the board stuck in circuit closed position causing the fan to never shut off. Stuck relays often cause death of uninterruptible power supplies. You could see if a new board is available for it. If you know someone handy with electronics, they might be able to fix the board. Ham radio operator, perhaps.

I think the humidity sensor was bad on my LG-made Kenmore.  It would run for a little while if I set it on 40% setting but wouldn't keep the basement anywhere near where it should be.

The part was available and cheap, but at 15 years-old I just figured it had done its time.

Who knows, maybe a mistake, with a new sensor it might have outlasted the "new" one.  I actually bought it used off CL, but it was only a year old, like new, guy had sold his house, moving out of state.