The Phantom Hum

I do a certain amount of freelance work, mostly for friends that have businesses. Last weekend I started work on an instructional video for Exerstrider, makers of poles you use while walking that really ramps up the exercise value. We were plowing through the script at a rapid pace when...

...The Phantom Hum reared it's ugly head.

I was using my basic rig, consisting of a Canon XL-H1 camera in HDV mode recording both to tape and a Focus Enhancements flash-card recorder, a Vinten Video6 tripod, a generic camera-top LED light for eye sparkle, and my Sennheiser EW100g2 wireless system. It was a bright overcast day, and even without any more light than the LED the pictures looked surprisingly good. We were halfway through a take when...something...didn't...sound....right. Owing to the low-budget nature of the gig, I was doing my own audio, and the headphones - Sennheiser HD280 Pro's, which I love - suddenly revealed a mid-level hum. I was running the camera off battery, and other than an Ikan monitor, nothing was powered by AC.

I did what you should do when presented with a puzzle like this: Start tracing the problem from one end to the other. I unplugged the headphones and plugged them back in again. Hum still there.

I unplugged the cable running to the wireless receiver and replaced it. Still humming.

I replaced the batteries in both the wireless receiver and transmitter. Still humming like a kazoo orchestra.

I was stumped. I stood beside the camera, scratching my head. Somehow in that time, my right arm brushed against the coiled part of the headphone cable...

...and the hum was gone. Take my arm away, the hum came back. If I grabbed the straight part of the cable, the hum remained, but by encircling the coiled part with my hand I could get it to go away. OK, monitoring problem. We soldiered on and managed to shoot the whole script before the weather went bad.

But the whole affair has been bugging me for two days now. The headphones have never been abused; I wasn't stretching the cable in any way; everything was as it should be. And The Phantom Hum didn't end up on the tape.

So I'm looking for advice. Have any of you ever had this happen? All theories appreciated.