Why would my Sennheiser Evo100 wireless flutter?
 

I've noticed on occasion that audio through my Sennheiser Ew100, when all is "quiet" has this flutter going on. It's quite audible on the phones and definately audible in the resulting audio. I've played with frequencies, checked squelch, used it in high and lower electric environments and it's stull there. Does anyone know why this might happen? It is a companding issue, or something faulty do you think?

Cheers
Aaron