I have the same problem but mine is a result of the fan blades hitting the shroud. Try gently squeezing and pushing your fan shroud with the engine running. If it gets louder or goes away then that's your problem. Some times we hit sticks and bushes or stuff like that, something my fly up from the bottom and cause you some grief. Check and make sure that all bolts holding the shroud are in place and not broken anywhere, this may result in the shroud becoming out of alignment. A severe overheat may result in the shroud heating up and changing shape just slightly. As an alternative you may want to convert to electric which is what I'm looking into now two rads and two shrouds and a water pump later.
Your engine needs and efficient cooling system and the shroud is a valuable part of that, No shroud and no cooling. It plays a big role.
Yes a mechanical fan does cost the engine efficiency, converting to an electric can safe you money but they're also more expensive too.
Converting to an electrical fan you'll need to make sure that you get the whole kit. You'll need the two fan configuration for maximum air flow, you'll need the wiring kit which may be sold separately and you'll need the temperature switch and their are two temperatures so make sure that you get the one that best suits your running needs.
Monday, December 1st, 2008 AT 12:30 PM