Buzzing 407 rotor tips
I had a strange thing happen yesterday and was curious to see if anyone knew what had caused it.
Tied a Bell 407 down to the helideck yesterday, normal construction, 60x60 ft, in the face of an oncoming thunderstorm. After it passed about an hour later, I went to untie the machine with a couple of guys based on the platform. Getting to the aircraft I noticed a buzzing sound but assumed it was the lighting round the deck. One of the guys untied a blade and asked me why it was making that noise. Sure enough, it was the tips of the rotor blades themselves making a buzzing sound like standing under high tension electric cables. We got a step ladder and there was no damage to be found, and no water coming out of the vent holes. The noise was coming from each individual tip, and was audible approx 6ft away from the machine. The battery was off, and the blades were all handled (carefully!)so presumably grounded, and the noise continued for a good 20 mins or so. Engineering guessed maybe a harmonic vibration from the platform, but never heard anything similar before.
Any ideas?