Neck Smell

Hi. I was working on a brown double Chicken stock last night. I used hind quarters for the initial stock and was using backs and necks purchased from the Rogers Park Fruit Market to double up last night. Weird thing: the meat smelled fine out of the package, it smelled fine browning it the stockpot UNTIL I would dump out some of the fat accumulating in the bottom, and then there would be an unpleasant odor. I pulled a couple of necks out and gave them a sniff, but they smelled fine. The smell went away after ahwile, but when I went to brown the next batch the same thing happened when I had to dump out some of the fat! The stock turned out fine in the end and it's in the freezer waiting to be used, but I was unsettled by the whole thing. The backs didn't give the same effect, only the necks. I'm definately gonna have to bring this to heat seperately and give it another taste before adding it to anything. Anyone ever have anything like that happen to them?