| ...I knew this was familiar. Look at this image: 
 See how the outer tread would push water into the center line, and the inner (more worn) tread would push it out?  I think your tires are bi-directional after all.
 
 
 I've still got money on your worn inner treads being the culprit.  Take a pic of the sidewall, if it's directional, it will have an arrow pointing "forward."
 
Recursively Yours,Kenny...
 PETZ Member #5
 
 
 
  You guys rock socks. (Click for pie-chart)
 
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