PCM to PCM lossless? / something is changing

[bomber]

Hi.
I did a little test.
Loaded into Avidemux 2.7.3 Windows 64bit an .mkv with 16bit/44.1 2ch .WAV audio.
I saved video as "copy" and audio as "PCM", exported keeping .mkv.
If I open the new .mkv file and demux the PCM audio and compare the audio to the original .WAV
I can hear the audio which was saved in Avidemux as .PCM has hiss added to it (particularly in the queit
bit at the start), and seems samples have changed, for example if you re-save a .WAV as .WAV and you check
in a program like EAC WAV Compare the samples usually stay the same, but saving PCM as PCM in Avidemux seems to change samples....
I have NOT changed anything in the filters box, eg. everything is unticked and set to none. See photo.
I do not know much about Avidemux, could someone please tell me why I'm hearing added hiss saving PCM as PCM?,
is it something like dither hiss?, is this truly lossless process?
*Yes I know if I save as "copy" samples stay the same, but why doesn't saving PCM as PCM stay the same?
Thanks for any info.

[eumagga0x2a]

Loaded into Avidemux 2.7.3

This is an unsupported old version, please update to the latest 2.8.1 nightly or at least to the last release (2.8.0).

I can hear the audio which was saved in Avidemux as .PCM has hiss added to it (particularly in the queit bit at the start), and seems samples have changed, for example if you re-save a .WAV as .WAV and you check in a program like EAC WAV Compare the samples usually stay the same, but saving PCM as PCM in Avidemux seems to change samples....

The internal audio format is 32-bit float, so conversion usually takes place as the sample format of PCM audio tracks is mostly of integer type.