noise floor is changing during recording

hello.

i was recording my synthesizer SY 85 into Apogee trak2 through coaxial digital cable using Sony Sound forge apps.

even though i didn't play SY 85, the noise floor level is changing

why did it happen?

the noise level was -69 dbfs, but suddenly it was changed to -79dbfs. then again it was -79dbfs

i live in south korea. and my outlet don't have ground wire ..therefore, i only use two wire such as -,+ for powering up equipment and computer etc..

is it because of ground wire?