I would hazard a guess at the DVDs making the scratching noise may be just a touch off center, whether possibly warped or just not quite fitting onto the spindle.
One other thought is the DVD lens may be starting to get a bit out of adjustment and making contact with the disc itself (would explain the clicking noise). You may first want to try cleaning the lens using a q-tip and a very small bit of rubbing alcohol (be careful with this as if you use too much alcohol it could completely mess the system up).