Drumming
I use a Mk3c helmet and have had no problems with it until yesterday when one of the visor brackets snapped. Luckily I noticed before I went flying. Its most unpleasant when it happens in the air! A chat with the others in the group revealed that this is quite a common problem with all marks of helmet. I assume its a fatigue problem due to buffeting. If you are being buffeted by the slipstream - is your seating position in the cockpit too high?