My car is making noise!!!
 

So, recently, my '06 Mini has started making a noise. Noticed it at freeway speed - at about 60, sounds like a intermittent hissing (sounds like air purging) that comes and goes about every second ant, at 30, about every two seconds. Does it while rolling, clutch in or out. Every once in a while, on smooth or any surface, when applying brakes, I can feel the antilock kick in. I disconnected the battery for an hour and reconnected it and the noise stopped for about 25 miles then started again. Any ideas??