Which drive is this? A stock Mac drive? If so, is it the typical evil LG drive? Or the Panasonic/Matsushita/Matshita drive? --- Or some other aftermarket drive that you added on your own?
Humming tends to be a vibration of the disc. I recently lost my favorite Toshiba DVD player. It started to hum, and you could sometimes see faint scratches along the edge of the disc. Worse, sometimes it would make extra sounds as the disc would hit the interior as it spun. It was a progressive error.
R.I.P., my Toshiba.
The disc isn't being held firmly in place is it should be, although not so bad as to be loose in the spindle. I tried to repair my Toshiba, but eventually gave up.
Again -- people often worry about how long a burned DVD will last, but as many of us know (or are learning), the drives give out much faster. In 50-100 years, we may have libraries of DVDs with nothing to play them.