Before I decided on firmware upgrades, I'd look under the heatshrink on both ends and make sure nothing is amiss. I'd also be looking VERY closely while it navigates the corners where the issue is occurring and see if the drawbar is lifting even slightly. My experience with sound issues just on curves has largely been the wiring of the drawbars or the drawbar lifting and bringing the sensor and IR LED out of alignment.
I'm not sure how you get the firmware upgrade that Mike mentions, I'd like to hear more about that. 
