Actually,this sound occured continully from start to stop the engine,but can hear clearly when the throttle position is from idle-30% . My camcoder cannot record enough details of sound because of lound noise environment that exceed the limit of microphone.
From the maiden flight video ,if you listen carefully at 4.20 mins.I used 50% of throttle and you will hear the sound like a frequency pattern.Ten of people in that day (as you will hear they said in this video) decided that this engine sound like a turboprop running with propeller. And at 5.25 mins.,you can hear this sound again.

I tested it on a bench stand before,but I ignored it.
I checked my Viperjet and confirm that everythings were fastened very well and no vibration.

Oh!If you still can't hear it.Please see this video at 6.20 min.It sounds like Anakin's race ship.
http://www.youtube.com/watch?feature=player_detailpage&v=5dcqnEFIw3A#t=382 Jetcat still believe in their QC,but this club has 4-5 jets that use P140RX and everyone comfirmed this engine have some problems.
Finally, if Jetcat cannot suggest me to repair it by myself.I have to send it back to factory and have to pay a shipping cost THAT it shouldn't happened with me.
I decided to pay more and bought this engine because Jetcat is a Hi-END turbine engine company and believe in their QC.The result is.......