Unwelcome noise

I'm just a beginer with REAPER. I have it on my laptop. Now I connect a mixer to the laptop and I wont to record. Evry time that I record ther is kind of a bazz or white noise in the background. I disconect the mixer and just click record without actualy recording anything, and clearly ther is a noise in the back. There was no electric devise neer it. How can I get rid of it? Thanks Ilan

Hello Ilan, is this noise present if you try and record with the laptop PSU disconnected? >

I tried it 

I had try with and with out the power supply Thanks for the quick reply :-)

How about posting an example? Buzzing and white noise are two different things.