KSP 1.10.1.2939
KSPIE 1.25.26

The RCS 90, or an RCS 45 which has been rotated to have one output pointing directly backwards, will make no sound when thrusting in the forwards direction, but will make sounds when thrusting in all other directions.

The RCS Perpendicular is not affected by this. Propellant used or Powered/Unpowered do not seem to affect this.

An RCS 90 or RCS 45 which points almost but not entirely perfectly backwards will make a noticeably diminished but still audible sound.