Speaker "whistle in "balanced" mode.

Have a solid state amp, tubed pre, and solid state one box player. Of late have been noticing an occasional high pitched whistling sound with amp switch in balanced mode. Only happening with one channel. Sound disappears when switched to single ended mode. Am currently using balanced ICS between amp and pre. Any ideas/advice appreciated.
Probably the early stages of one of the tubes in the preamp going bad. Since the whistling only happens in balanced mode, try switching the tubes in the balanced part of the circuit and see if the whistling changes channels. From what you are describing, this is the most likely problem. If the whistling does swap channels with the tube change, you can change a single tube or for best results, replace all tubes in the balanced section of the circuit.
Px25: Swapped out the tubes, and whistle now gone. Thanks so much for your time, and advice.
You are so welcome. I only wish all problems with audio and life could be so simple. I hope you continue to enjoy your system.