Tascam DR40 white noise, without microphone plugged

Hi Elektrik people !

I’m on something a bit confusing,

I bought some days ago a new TASCAM DR40 and it looks like It is a problem on…
I can hear some noise and control his volume with the input volume button… No mic plugged, in XLR or line position, when the “pre rec” or “rec” is on…

I can listen a click each volume step… and the noise and clicks grows as volume…
So when i’m on 100 it’s like it is raining …

I do not understand why…

Is it normal ?

Thanks