recently purchased a pair of C18's primarily for flying and worked well on lowering the background noise in the cabin. However when repositioning one of the ear buds, I would always always get a medium pitched whistling in the ear where I think the system is trying to hunt down the frequency(s) to cancel, as I cover the microphone on the back of the ear bud. It would die away after a couple of seconds and it was not ear splitting but annoying none the less.

So has anyone experienced this with thier units or do I need to recalibrate them in some way (with no visible parts for changing!). To be honest for the cost, even one long haul flight they was worth it but keen to get more good use out of them if I can for a while