<VV> Bad sound when trying to start

[Kinzelman, Andy]

Guys,
I drove my car almost 200 miles yesterday - it ran as good as usual.  Started & drove fine to work today.  
However, when I tried to start it at lunch, it cranked OK, until it made a thunk/clunk sound  - then stopped cranking.  I tried it twice more; those times I could hear the solenoid kick in, then immediately 'thunk' again.
I looked underneath (thinking the bellhousing bolts may have loosened up).  Everything looked normal.  There was no reasonable way to look at the starter mountin in the lot.
I tried one last time.  This time it started up as it normally does.  I drove it home, parked it, & tried cranking again.  No problems.

Any guess as to what caused that problem?  It makes me leery to drive it any more without diagnosing it.
Thanks,
Andy K.