Possibilities:1) The laser diode or possibly some other part is failing.
If number 1) is the cause you’re out of luck.2)) The laser diode is dirty causing the recorder to misbehave.
If number 2) is the cause try a cleaning disc, now is a good time while the recorder still recognizes playback of discs.3)) The spindle is dirty causing the disc to slip.
If number 3) is the cause you may try a procedure written by member jjeff and used by many members on Panasonic recorders. There was a member that tried this procedure on a Pioneer with 100 percent success. The member used isopropyl alcohol instead of saliva.
Eject the disc tray. Unplug the machine from the wall. Gently slide the disc tray until you can see the spindle inside the drive(not sure if this is possible on a Pio, but I'm just mentioning it). Then using a long handled q-tip, I like the wooden handled ones like used in the medical industry or VCR service of yesteryear. Wet the q-tip with some type of isopropal or tape head cleaner. Using a flashlight to help see, gently clean the spindle the best you can. The spindle part that gets dirty on the Pannys is the lower part that holds the center part of the DVD. I gently spin the q-tip between my fingers while resting it against the rubber spindle. After you're done wait a minute for the spindle to try, plug the machine back in and give it a try. It works for the Pannys, maybe it would work for the Pios?
I decided to take the 'saliva' idea mentioned above in this thread, and instead use isopropyl alcohol. I coated the inner circle, transparent section (the part that gets 'concentric rings') with a light sheen of isopropyl. I then put it in the tray, activated the 640, let the disc spin around there for a bit, and removed it. Then I left the drive tray open for an hour to dry.
And just now, I've finished burning/finalizing several DVDs, including one new Sony 16x DVD. Absolutely no grinding whatsoever. It burns as quietly as it did several months ago!
Thanks everyone for the help. I will not miss that 'grinding purr.'
Some folks will tell you to use T-Y 1x to 8x discs claiming they are easier on the laser/require less laser power. Might be worth a try, I don’t believe the T-Y use less laser power but I believe because they are sticky – they tend not to slip after other brands start to slip. BUT
I would try using Verbatim AZO DVD-R 1x to 16x discs. Many including myself claim that these discs work best for real-time 1x burns and are very easy on lasers.
Lastly, try cleaning around the fan, making sure there is no debree blocking air-flow and make sure your unit is not stacked, blocking airflow and make sure you don’t have any components stocked on top of the recorder – this can cause moving parts to go out of alignment and also can cause heat related problems.