How can I diagnose this problem on my '04 Jetta GLI?

I have an '04 Jetta GLI 1.8T and the check engine light is on. Naturally, I checked the oil and oil filter, and I also replaced the air filter because I had a feeling it needed to be done. The light is still on, and every once in a while (about three to five times a week) there is a whirring noise under the hood, almost like a vacuum noise. How can I diagnose what the problem is? If it helps, there are about 57k miles on the car. Thanks a lot in advance, everyone! :)