I'm not sure that the speed should increase, but I believe that the system should automatically switch to "recirculate" so that the blower system doesn't suck in exhaust fumes while you are reversing. You may be hearing that.
Is this on the later vans?sounds like a good idea.i hate the smell of the screen wash.im sure some gremlins keep topping mine up cos ive never put any in and it still stinks
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