I have a problem with a Bell Sweep helmet that I haven't had in over 25 years of riding with other bicycle helmets. There's something about this helmet that causes it to make an intermittent "squeaking" sound. The noise comes and goes, but it seems to be worse in the winter than in the summer. I've tried tightening and loosening the strap that fits around the back of the helmet. I've also tried moving that strap up and down on the other strap that allows for vertical positioning. Nothing seems to eliminate it totally. I can not replicate the noise by holding the helmet and twisting pieces around, so I can't identify where the noise is coming from exactly. Any suggestions? (Sorry, riding with an iPod is not an option for me!)

Something rubbing against the foam? Or the shell might not be flush.