This is on a Maytag MDBD880AWW dishwasher ( will be on many other Maytag dishwashers as well ) 

It was making a grinding noise only when draining.

I thought it was the drain impeller except that it was draining, but making a huge grinding sound only when it was draining.

It was a broken upper discharge housing, which we bought new one.

The broken off disc will float up during the wash cycle and not make a noise, when the dishwasher is draining the broken off disc will drop down and hit the wash impellor making the noise. 

#11 on breakdown below...

Thank you kelly for the information and pictures! :-)