The same tip I give to BradleyW applies here. Are you sure the screws in the ek top are not to over tight? We all tend to screw those things all the way and exerting too much pressure with the idea of sealing really tight the top and avoid leaks (In fact the sealing is made by the o-ring and no need to exert too much pressure). However, in doing so the top can push down on the impeller creating a gurgling sound, specially at high speeds, with the side consequence of also hindering flow. So test outside your system and try to loose a little bit the screws to see if that solves your problem. It worked to me and I learned in the hard way. The tip was given to me by people at FrozenCPU (Geoff).