On Sun, 29 Jun 2008 08:57:09 -0700 (PDT), "
wrote:

Got a Cessna 206 that emits a steady tone during transmission only.
The pitch increases with rpm to a point where ATC refuses service. No
tone is emitted on battery power only (Alt. off). No problem
yesterday, problem today. Noise remains with audio panel on/off, using
L/R trans switch, speaker on/off, swapping headsets(Bose). Aircraft
has 95 amp alt. (belt drive), noise filter. Please email me if more
infor. is needed.
Thanks
R
Sounds like a bad noise filter on the alternator.

rj