Inspect inside the grate and feel around the impeller. Look for debris. Check for movement on the shaft and impeller side to side. If there are larger pieces missing off the impeller then don't operate. If it is just dented then it should be ok to ride for awhile. Some members might rent you the spline tool and you can grind the dents out of the impeller or buy a new one.
The impeller is small diameter so a little imbalance is ok.