Playback() .alaw

hi,
when Playback() function plays .alaw audiofile, in the beginning and at the end i can hear some kind of “click” sound.
BUt if i play .gsm file its sounds good.
I use asterisk 1.8.4.2

Suggests there is a DC offset in the recording. What codec is used for the actual call (i.e. is there any transcoding).