Do you mean it hums before you put in a disc? That should be the fan in the enclosure (which could be worn or dirty).
About the media recognition, is this with both CD and DVD? You'll need to run the Plextor self-test with approved media, as described in the manual or on their website. There's little you can do if it fails, unless you want to start fiddling with the laser power calibration (hit & miss).