Many people who stutter experience a reduction in stuttering frequency and severity when speaking in the presence of noise. This is a group phenomenon, but it's not exhibited by everyone (like me, I have a harder time talking in loud environments). Like others have suggested it might be because you're not getting as much auditory feedback--similar to how delayed auditory feedback works. Theories would suggest that because you can't rely on auditory feedback you rely more on proprioceptive feedback (knowing where your mouth / tongue are in space relative to one another). Alternatively it may be that the reduced reliance on acoustic feedback requires you to rely more on "feed forward" models of motor control.