Plantronics Headset and Squawkbox 4 question

Approximately 50% of the time when the mics are keyed within squawkbox 4 (FS9) I hear a noticable "chainsaw starting" burst before voice comes on--the voice is perfect but it's that annoying micro-burst of noise when mics are keyed at the very beginning that causes you to jump. The other half of the time, everything works fine. I was wondering, what would cause that noise to happen in half the cases? I try to duplicate exactly the steps I take when there is no noise and it seems completely random. I've tried reinstalling SB-X Fi extreme sound drivers etc, and squawkbox, but nothing seems to help. Is there a specific "headset" setting that I need to use in windows? I noticed it gives me the options of CD, DVD, FM, AM tape recording quality and 1 or 2 channel 16 bit. For the plantronics headset I use the wave setting vice direct.. I've changed that too and can't duplicate.Any assistance would be great.ThanksRich Perry(i7 965 extreme 3.8Ghz/6GB OCz DDR 3 RAM/Nvidia GTX 285 2GB video/SB X-Fi xtreme audio/Vista Ultimate 64bit SP2/Raptor 10000rpm HD)