Are you sure the trans is really "gearing down"? My '90 and now my '99 LS made me think the same thing at first. For cars I was used to (Fords, CHryslers), when you let off the throttle at freeway speed they allow the torque converter to unlock, so you essentially coast with little or no engine braking. The LS's do not unlock the TC until below about 50mph. It really does feel like a downshift when you let off the gas. Then, at just below 50, the TC unlocks and you coast w/o any engine braking. I learned to like it when I got used to it.
Sorry if I am off-base, just my own observation.
Gene
Howdee! I have only had this car a few months. I thought it was the torque converter, too but it really makes a lot of noise. It does this @ any speed...especially when you drive it like you are racing the INDY500 (like my husband does)...I am going to check the fluids first then possibly check the throttle...we shall see...thanks for the info!! :D :D :D
