I have a 2001 Subaru Outback that has a problem i can't figure out. During a snow storm i tried to downshift easy so the car wouldn't lose traction, as i did a loud thunk noise came from underneath. The car just built revs without moving very fast, i thought it was just a clutch. Two new clutches later, it still does it. Mind you that after each clutch was replaced the car drove fine for a day or two then would start doing the same thing. I was told it might be a viscous clutch or differential, can someone put me at ease?