The grinding noise is the result of a buildup of dust on the rubber disc that your BD sits on. When there is too much debris on there, the rubber won't grab the disc and the grinding noise is the motor spooling up with no disc on it.
Blu-Ray Discs have a scratch resistant coating on the bottom that makes them more slick then DVDs, which is why the noise sometimes seems intermittent.
Here is how to fix it:
Remove any disc inside the player and disconnect all cables- especially the power cord.
Use a #2 philips screwdriver to remove the four screws on the side of the BD player's cover. Then remove the three screws on the back. Slide the cover toward the rear of the BD player and remove.
Inside you will see a black box in the middle where the BD is loaded. Use a #1 philips screwdriver to remove the four screw on the very top of the black case. Depress the tab on the back of the case (Sort of off-set to the right) and the cover will come off.
You will see the rubber disc and it will probably have some dust and other debris on it. Using a q-tip doused in alcohol, clean the disc until there is no more debris on it.
This will last you about 6-12 months until the noise comes back.
Hope this helps.