johnson pumps, eh?
i'll shortly be using the CM30 i found at a local surplus house.
3.5m head is really nice for its compact size, but it's a little on the noisy side. Kind of a high pitched whine. I'm also a little concerned about RF noise the brushes might introduce into the system through the 12v line.
But you didn't say what model you're using, so this could all be rather irrelevant. Care to elaborate?