IOS audioToolBox 编码AAC_HE 有些爆音的问题

最近公司想将audioToolBox硬编码率从128K,降到64K,之前使用的是LC的编码方式，64K码率的fdk_aac的软编aac_he音质很好，但是公司线上已近不用软编了，我试着配置了硬编的参数如下：

    AudioStreamBasicDescription in = {0}, out = {0};
    in.mSampleRate = 44100;
    in.mChannelsPerFrame = 2;
    in.mBitsPerChannel = 16;
    in.mFormatFlags =  kAudioFormatFlagIsSignedInteger | kAudioFormatFlagsNativeEndian | kAudioFormatFlagIsPacked;
    in.mFormatID = kAudioFormatLinearPCM;
    in.mFramesPerPacket = 1;
    in.mBytesPerFrame = in.mBitsPerChannel * in.mChannelsPerFrame / 8;
    in.mBytesPerPacket = in.mFramesPerPacket * in.mBytesPerFrame;

    out.mSampleRate = 44100;
    out.mFormatID = kAudioFormatMPEG4AAC_HE;
    out.mFramesPerPacket = 0;
    out.mFormatFlags = 0;
    out.mChannelsPerFrame = in.mChannelsPerFrame;`

发现输出的AAC文件播放起来一直有些轻微的噪声，谁有相关的经验可以帮帮我嘛，万分感谢。