the backgroung noise from rtp stream

[Soho Soho123]

Hi all,

my pipeline is
appsrc-->decodebin2-->audioconvert-->audioresample-->alsasink

I use a gstbuffer that created by
buffer = gst_buffer_new_and_alloc (ONE_PKT_SIZE);
then use buffer for socket system
readval = read(run_thread->netfd, GST_BUFFER_DATA(buffer), ONE_PKT_SIZE)
then
GST_BUFFER_SIZE(buffer) = readval;
finally ,
I call ret = gst_app_src_push_buffer(app->src, buffer); to push buffer
to appsrc,

It can work to play rtp stream , but the background noise is loudly.
even I memset GST_BUFFER_DATA(buffer to 0x00, the noise is loudly,

Does anyone have idea why there is a laudly noise when play rtp stream?

Thanks!
Soho