Hi
I too use the overcovers, even when under the covers.
Are you sure that the rustling was friction ON your mic, rather than friction of the fabric generally, that is being recorded by your mic.
I always use a fluffy overcover on both sides of the mic, and ensure that the top of the mic is also covered by a undercover.
I also tape a generous loop of the cable so theres def no pulling on the mic cable causing problems too.
Peter
Post Production Dubbing Mixer