The output <e1>noise</e1> was caused by the biasing <e2>resistors</e2> and the shot noises of the base currents of Q1 and Q4.
The output noise was
The output noise was caused by the biasing resistors and the shot noises of the base currents of Q1 and Q4.
<triplet> noise <subject> resistors <object> Cause-Effect(e2,e1)