Phono preamp hum

Does that unit have an AC filament supply?  If so, I could imagine that there is a leak of AC into the signal that takes time to occur when you start from cold, because something needs to heat up before it leaks. It's important to know that this is a real "hum", meaning 60Hz or 120Hz pure tone.  If not, we will start over.

I have no idea if it has an AC filiment supply.  It has a wall wart power supply.  Once it starts humming, it gradually gets  louder and I have to unplug it to stop.

Have you checked the grounding? Make sure the ground wire is tightly secured. Then try removing the ground wire. Is your linestage plugged into the same outlet as the Bellari? If not, make it so.

Yes, the ground wire is secure.  I have tried grounding it a few different ways with the same result.  Like I said, it sounds fine for about 30 minutes, then the humming gradually becomes louder the longer it plays, or even not playing.