1998 Monte Carlo Starting Problem

1998 Monte Carlo with 110,000 miles on it. No car problems at all since we've had it, but now it has been slow to start recently. The other night I was away from home and tried to start it and it just made a humming noise in the engine when I would try to start it. The lights, radio, wiper, etc all work. There was no clicking noises at all. It finally started after 6 or 8 tries. So, I drove it to the garage and left it to be worked on. I just spoke to them and they said it has been starting fine and nothing comes up on the diagnostics so they can't do anything to it. I assumed it would need a new starter, but they say it doesn't. Anyone have any other ideas?