Vibrating in rear on the drivers side?

[2002MineralGray]

Ok so this morning I was running VERY late for class at my college so I didn't have time to warm up my car. When I came outside it was freezing and my temp gauge on my rearview mirror that it was 11 degrees outside. I started the car and only let it run for about 5 minutes before taking off because I was so late. When I pulled up to an interesction I heard this low pitched vibrating noise from behind me. The only way I can think to describe it is when you hear a large semi or tractor trailer as it down shifts and slows down they make a low pitched humming noise. So I looked in my rear view mirror assuming one was pulling up behind me but the street was empty. Then I heard it again so I shut off my heat since it was on full blast and I wanted to know if it was comming from my car.

Sure enough it was and I got really worried. The when the light turned green and I began to let the clutch out the noise became more notice able. I have NO CLUE what this noise could be but it went away after the car warmed up a little bit from me driving it. It was a low humming noise that sounded like it was comming from the rear driver's side. I know my description is vague but could anyone lease help me out because Im worried there is something wrong with my car. Today was the first time it EVER made that noise so I assumed it was from the really cold temperatrures because when I came out of my last class it was about 40 degrees out and the car ran fine with no noise. Sorry for the long post but I wanted to be as specific as possible.