When I was trying to start the bike last time, I heard a grinding noise. Now when I run the engine a push the starter while the bike is idling, I can hear what sounds like ball bearings bouncing around. If it's not the started what else could it be?
What you are hearing is an over-running clutch. It keeps the engine from driving the starter if the solenoid were to get welded contacts. It is not necessary or prudent for you to test that in the manner you describe. But yours is working as designed.
I haven't replaced a 1500 starter, but the 1100 starter has nothing but an o-ring holding it in that crusty bore once you remove the bolts. Spray some lube in there.