Are your battery cables tight? Grounds all good? Even if the bearings were out of the alt it wouldn't be grinding until the engine was running making the pulley move.
Whed they tested your starter did they bench test it or just pull it off the bellhousing-ground it- and hit the key?
This sounds wierd but turn the headlights on, have someone watch them or do it at night, and hit the key. If the lights go out your battery is going dead or your losing your ground. Either of the two situations would cause you to loose the amps needed to turn the starter over..which can cause a grinding-rapid clicking noise.
Double check the starter selonid, good posibility the noise is coming from there....