UR824 Preamp Hiss

I am recording line level input from my TC Electronic outboard gear. Preamp is set to minimum level and pad switch is engaged and Im recording signal with noise floor at around -78db , when input meter is -1 dbfs

When no input is connected and preamp is turned all the way up , i can hear hissing noise.

I had the electrician check my power supply and grounding seems to be fine.

Any idea why the preamp makes hissing noise ? or am i simply hear the noise floor when preamp is boosted all the way up ?