Very weird audio sound

Hi,

I have a video chat program in Flash, and the video/audio works well but with some guy, the audio is crap for him. I can hear him well, but when I talk, he hears very weird sounds, and this sound echos, even if I use headphones. The echo isn't really the problem here, but what he hears. It's some kind of "TAK-TAK-TAK-TAK-TAK-TAK-TAK-TAK-TAK-TAK-TAK-TAK-TAK-TAK-TAK-" sound...

Any ideas?

Thanks