There should be a cover bolted to the left side of the box which I believe is where the power take-off would be if so equipped. Remove this and look inside to see if you can see anything. If you can get your finger into the bottom of the case you can poke around for loose bits of metal.
Regardless, your trans is going to have to come out to be repaired. It's supposed to turn smooth and free, anything else is bad news. Transmissions have this bad habit of not fixing themselves.
