Posted for "John Chambers" <fly2fast@cableone.net>:

 Ian,
 
 Long ago, I encountered the same problem while testing my new Vari-eze.
 The dynamic mic of the now-old David Clark 10-30 headsets could not
 self-cancel the increasing noise as I approached cruise speed.  After
 cussing my radio and intercom squelches (which worked perfectly) I tried
 the electret mics from the DC 10-40 headsets.  That solved the problem
 forever and I still use these 20 year old headsets.  That may not be
 your problem if your headsets are relatively new, as it is now difficult
 to even find a headset that comes with the cheaper dynamic mic.
 
 John Chambers
 

"""
 Up to 175 knots my radios and audio panel function perfectly.  At about
 175knots plus I start to get a lot of noise through the headset  as if
 the mic has opened and let the aircraft noise into the system.  I have
 tried playing withthe squelch to no avail.
"""