I have always wondered about this. The universe is literally packed with radio signals. Most of which are perfectly normal, but near impossible to distinguish from a lot of other things. To be honest, the only truly unique noise in the universe is found here:

https://en.wikipedia.org/wiki/Bo%C3%B6tes_void

Almost 250 Million Light Years of absolutely nothing. And it was found purely by accident. This is easily one of the five most mysterious locations in the universe. 250 Million Light Years is just mind boggling huge. For a place that large to be this empty is nothing short of unbelievable.

I think that Michio may be correct about alien transmissions. They could be utilizing such a broad bandwidth to speed things up, that a couple of bits are all that exists at any given location.