Can the harmonic balance come loose?

The crank will have to come out to replace the thrust bearing.

I might have come off a bit harsh in my post above, but I'd say 95% something is screwed up with the thrust bearing. Try tightening the balancer bolt if you like, it'll only take 5mins but don't go crazy. If it doesn't budge then you've got bigger issues and you certainly don't want to go swinging off that thing and possibly snapp the balancer bolt (that'll require some force though).

Why this has happened on an engine with only 10K on it, I can't say.

edit: funny enough a friend's 318 used to do the same thing. In neutral he'd give it a rev and we'd watch the crank move back and forth at least 1/8th of an inch