A very hard "thump" comes from the rear end on upshifts when under less than full power. I was told by two different mechanics that it was due to a vacuum leak, and one determined that the leak was inside the dash area. The independent shop advised me that no real harm was occuring (fluid in the torque converter), but the dealer's mechanic stated otherwise. This car has over 300,000 miles on it and other the above problem runs fine, see no reason why it won't hit 500,000. Would really like to hear from another source on whether I should go through the expense of a dash removal to fix this problem, or if it is a relatively benigh situation.