i've been dealing with the same thing recently as I gear up to shoot a digital feature. using a sennheiser 815 and a wireless, both quality mics, i still hear that noise. here's my "clever" workaround...(posted 8758 days ago)i record a patch of audio with the mics turned off but at the rec. level i plan to use. then i record the footage. i then digitize the tape and export the silent audio clip plus the audio footage as a single wav file, bring it into a PC program called Cool Edit Pro, where I sample the silent bit (that still contains the camera noise) and apply a noise reduction filter to the entire piece. this removes the camera noise frequencies and leaves the rest of the audio alone. the result is amazing- super clean audio, minus the noise. i've only tried it on a clip or two but it worked great. it's a rather lengthy process requiring mucho equipment, but it's worked so far.
hope that helps!