Here's an idea; four arrows moving right, towards a line. The line has a hole in it, only allowing one of the arrows to pass through. Demonstrates filtering and doesn't have any accidental significance from symmetry.
There's a somewhat more simplistic version of that on the above-linked results, which is four centered, horizontal lines, each decreasing in width from top to bottom, which I think communicates basically the right idea: you have a much of this thing up here, but you can get less of that thing down here by clicking this (maybe). Conceptually speaking I like it more than a funnel, but it still needs a text label to convey the right thing.
There's also a literal filter icon, which could only ever work at somewhat larger sizes due to the mesh. Without knowing that the context of the search results is that of "filter", however, you probably wouldn't know what it was by simply looking at it.
