Take the cover off , see if it still hums , if it does , then listen and poke around trying to find some thing that is lose , some type of fastener etc.
If you find that its vibrating at the brush and all the fasteners at the brush area are tight .
Then remove the brush holder , maybe with some magnification to see if there is signs of arcing between the carbon brush & the brass holder , I have repaired some were the glue holding the brush to the holder after some load melted the glue and it lost contact - no current to run load , in your case it could be lose and transferring current but making intermittent contact creating the hum .
For the last 15 amp variac , I priced the brush & holder and they wanted $90 or about $30 if they but new brush in my old holder .
I thought that was too much for what was there and I did not want to be with out the use of it for sending mine in and wait to get back .
It can be very tricky if it is still stuck like mine , the glue was still holding but no continuity between holder & brush .
I was able to get the brush out , without breaking the carbon brush and then clean the old glue off - what remained was a ruff surface on the glued side [ without the glue ] so I filed the notch in the holder until I had a bright surface where the holder had held the brush , then floated a lot of solder and kept it liquid and positioned the carbon brush inplace and let cool , then put all back together and tested with some equipment including a motor .
And all is good .
Humming in AC equipment , most of the time there is something lose , like a fastener somewhere .