Had it on and off for a few weeks. Now it's there all the time, as soon as you turn engine on. It kind of fades in and fades out, fades in and fades out, every 1.5 secs or so. Seems to be down the nearside side of the engine block, about halfway down, not the top of the engine. I may be imagining it, but it seems to be accompanied by a very faint Darth Vader-like breathy roar from within the engine. Anything to do with the water pump?