What is wrong with my Mazda 3?

[Crystal]

I have a 2004 Mazda 3 with only 80,000 kms on it. My problems started about a month or so ago, my car would have problems starting right up, it would turn a few times, shake a bit, knock 3 times then start.. That was not a consistant issue, it happened maybe once or twice a week. I didnt think much of it because it was not too bad at that time. Yesterday i was driving my friend home, and we got to a stop light, and my car starting to shake and vibrate really bad, it there was a very loud humming noise from the front, i basically thought my car was going to break down right there, but then the light turned green and as soon as i stepped on the gas, the noises and the shaking stopped. When i got down into 2nd gear again, it started up again. Seems like this is happening when im in low rpm's or idling. There is also a small constant thumping under my feet and the passenger side feet as well when this is happening. As soon as i step on the gas, everything quits, but as soon as i let off gas again, it starts again. Also, when i turn on the A/C sometimes it gets worse, sometimes it stays the same. I have tried talking to the Mazda Dealership and they said over the phone that its probably just a spark plug.. it just seems a little worst than that.. has this happened to anyone else who's owned a mazda 3? What could this possibly be? and how expensive would it be to fix?