NAD 326 headphone noise issue ?

I'm guessing that the headphone jack does not have a mute circuit when turning the amp on/off.

If so than what you are hearing are the noises that amp's typically make during turn on/off but which we are spared from hearing (through the speakers) as most amps mute the speaker outs. The clicking sound that most amps have after a few seconds of being on {or immediately upon turning the amp off) is the sound of the relay unmuting or muting the speaker outputs.

Thank you.

Is this problem for the headphones?

Dok81

Some integrated amps doesn't have a well designed headphone output section, that's probably the cause of your issue.

Which headphone models are you using?

I use Senn EH350, but i tried an Onkyo DP-400.
The NAD support center wrote that is not normal, but i found other users who has this problem too (on the speakers everythings right). The 2 new amp (which i tried) serial number very different and were different dealers.

Thank a lot!

Dok81