I have a 2001 330i w/5 speed automatic tranny. When accellerating at 50%+ throttle, I get a rumble-strip like vibration, usually between 2500 - 4500 rpm, in 1st & 2nd gear. At first, I thought left rear tire was flat, but no, OK, fully inflated. 112,000 miles on tranny, could this be a problem with clutches; or could this be a problem with the differential? Any ideas?