* Fan Clutch Operation: The fan clutch is a device between the water pump and the fan. It allows the fan to spin freely at low engine speeds (reducing parasitic drag) and lock up at higher engine speeds (for increased cooling). The clutch engages based on engine temperature and speed.
* Reverse Gear and Engine Speed: When you're in reverse, the engine speed is generally low and irregular. This is especially true if you're slowly maneuvering or backing up a hill. The fluctuating engine speed can cause the fan clutch to cycle or even partially engage and disengage repeatedly.
* Worn or Failing Fan Clutch: If the fan clutch is worn or failing, it might not disengage completely at low engine speeds. This could lead to the fan blades getting close enough to the shroud to make contact, especially during the inconsistent engine speeds of reverse. The fan might be able to freely spin at higher speeds in forward, but the inconsistent speed of reverse causes the clutch to partially engage and bring the fan closer.
* Other possibilities (less likely): While less probable given the specifics, other possibilities include:
* Misaligned Fan or Shroud: Although less likely to only occur in reverse, a slightly misaligned fan or shroud could exacerbate a problem with a partially engaged fan clutch.
* Bent Fan Blade: A bent fan blade could make contact with the shroud, and might be more noticeable in reverse due to the fluctuating engine speeds.
In short: The most likely culprit is a failing fan clutch. You should have it inspected and likely replaced. Continuing to drive with the fan hitting the shroud could damage the fan, shroud, or even the radiator.
