Have you modified your VX lately? What speed are you going when you let off the gas and the sound occurs? I ask because after lifting my VX I hear the same sound when I let off the gas but only at speeds above 40mph and the rpms drop to around 2k. This is apparently a common issue after lifting and has something to do with the driveshaft angle. Best remedy right now is to grease the three points of the driveshaft. Hope this helps