I checked the noise floor of multiple recordings I took in the same day in the same room with the same set up. I recorded 10 different tracks all peaking around -6. 4/10 of these tracks had a noise floor of -85 while the other 6 all averaged around -59. I can't think of any reason for this other than the stopping and starting of the PC's fan (no I can't get this out of the room).
Also taking different readings of the same track yielded different NF levels. If I selected the whole track (with noise floor at the beginning) the NF level would be sometimes significantly lower than when I was just measuring the vocal. Maybe the fan was kicking in when I began to talk? Sometimes it would be a small difference. If I am having an issue with varying NF levels (most likely because of the fan), I should only be measuring the vocal, or the parts I will ultimately use, correct?
I use a c1000s akg, and Focusrite Scarlett 6i6 USB Audio Interface.