I repeated my experiment to get noise level. To get that I disconnected the bandpass filter SBP-10.7 from channel A of network analyzer AG4395A and terminated both the open ends (open end of filter and open end of channel A) with 50ohm terminator.
Reference level had been corrected, signal and noise data had been collected separately w.r.t that level.
Command for GPIB: ./netgpibdata.py -i 192.168.113.105 -d AG4395A -a 10 -f filename
The result is as follows