Pop/Click Noises while using Turntable through DDJ-RR in Rekordbox

Hey folks,

I brought a used DDJ-RR in superficial top state a month ago and use it in combinatinon with Rekordbox 5.0.3.. (Original buy date: 02-01-2017)
The mixing of digital WAV files works fine without noises.
(PC Mix, Sound output through computer)

Problems:

1. 
While using a Audia Technica AT-LP120-USB (Setting: Line Out) with my DDJ-RR (Setting: Line In, PC Mix: On) a clicking and popping noise occures like the Vinyl is dirty as helll. But I tested it with several vinyls and they are all clean. The noises still occured.
While using the same vinyl's but sending the sound directly through the Master Out (cinch) without usage of the PC/Rekordbox there aren't noises. The sound is on point.

2.
While using the headphones for cueing there occures a noise like the buffer is to small. But the setting of the buffer should be big enough I guess. There are still resources untouched.
(Occures while usage of any kind of audio file)

Technical information:
Processor: AMD Phenom X4 955, 3200 MHz, 4 cores
RAM: 8 GB DDR3
Mainboard: Gigabyte GA-770TA-UD3
Graphiccard: ATI Radeon 5870 HD
OS: Windows 10 Pro
No additonal/external soundcard installed

I am actually helpless because I follwed the instructions exactly and couldn't find anything about this issues in the internet.
Did I missed any settings in Rekordbox?

Thanks in advance for any help, advices or suggestions I receive from you guys!!