Reply to Interference with wireless headphones ?

Hi ! I just bought wireless (bluetooth) headphones (Sennheiser MM 550-X).And there's something weird.
When i am outside (and only outside, not in a building), and that i turn my head (to the left, while walking, with tracks from my iphone playing), there are some interference.
I never have this problem when i am in doors.

What is it? a problem from the product, or normal interference because of all the waves (or i don't know) from the city?