Turntable Hums after an hour

I have a Pro-ject 1.2 turntable which, after about an hour of playing, starts humming from where the pulley is.  Took it to a repair shop but he couldn’t duplicate the problem. He tested it with the belt exposed and platter off.  I know nothing, but when I duplicated his test with the platter off, it took longer for the hum to show up, but it eventually did. It seems to happen when the plate where the turntable pulley is attached gets really hot.  Any ideas what it might be. Repair guy cleaned it and said everything inside looked fine. Going to take it back to him but he seems stumped.