First major problem..... hopefully the last!

While coming home from work today I noticed a popping noise while coming to stop at the stop sign and I thought I was hearing things.... so I rolled down the windows and I heard it louder as the sound reflected off the building.... Also I hear the same thing when I put it in reverse and also when I put it park and the car slightly rolls to a stop. Has anyone had this problem so I can go in there and tell them that this maybe the problem and I can get it done ASAP. Also, if you might think of what it is I would appreciate the response. Thanks