Noise (and visual) pollution caused by the neighbours' kids on their trampoline. At its worst when you're trying to enjoy a quiet sunbathe in your garden sanctuary but these screeching little humans repeatedly bounce above the height of the fence, possibly even making eye contact as they do so, and definitely destroying your privacy. Extreme cases of trampollution can occur when your garden is sandwiched between two gardens which each have trampolines. It can then feel as though you're sitting - perhaps also trying to read - at the heaving centre of an adventure playground.
I really fancy kicking back in the garden with a book and big dooby, but I'll have to stay inside because of the currently high levels of trampollution.