(Note: to skip straight to the solution, read the last paragraph.)
I live in an apartment complex where I'm simply afraid to call maintenance because of their complete incompetence: on their first visit, one guy almost electrocuted his pal trying to fix light sockets. On their most recent visit, they practically broke our air conditioner. So when our dishwasher was making grinding sounds, I decided to investigate.
The noise was coming from the bottom of the dishwater, so I removed the kickpanel and listened around to isolate the noise. After some time, I concluded that it was the motor. More research on the internet. Yes, motors do go out on dishwashers... they make a grinding sound... but something wasn't adding up, because my grinding noise was intermittent; it happened during every cycle, but it comes and goes... sometimes louder, sometimes softer.
The final conclusion I made was that I had broken some glass in it a month back and there was still some glass in there somewhere banging around. This was confusing, since we had only been recently hearing the noises and the sound was not coming from the tub, but from underneath the tub... so, I took out the bottom basket, unscrewed the filtering thing that catches large objects/food, and what do you know.... sitting at the very bottom of the hole where the water gets pumped was not glass, but a cherry pit!
This makes perfect sense since a couple weeks ago I had eaten a bunch of cherries and threw all the pits on a plate. When I scraped the plate, I must have missed a pit that got stuck to the plate, and into the dishwasher it went. the pit was small enough to slip through the filtering grate at the bottom of the dishwasher and, as the water is getting pumped through the dishwasher, it's perhaps hitting the pumps propellers (?) causing this grinding sound.
So long story short: before you go spend lots of dollars on a service call, just roll up your sleeves, grab your power screwdriver, a strong light, a fan to keep you cool, and see what you can see. It may just be something as simple as a cherry pit that's causing all that racket.