Check to see if you have any white balance shift (not sure if there is an option for this on the Rebel), 2nd what is your 18% target? Grey card? is it filled the center of the frame?
You cannot shift the balance on a rebel. The 18% target is a custom target we use for all of our cameras. In total we have over 60 Canons in the field. Yes - card is always framed full.
What is adding to the complexity of this issue is the fact that this is not consistentyly reproduciable.