90,000 change in acceleration sounds

I have an '05 Prius with 92,000 miles -- I live in Wisconsin and drive 40mi to work in each direction.

Recently (last month or so), I've noticed that the normal sounds I've always heard on a relatively hard accerlation (say, from a stop) have changed. I usually heard the engine start, rev a bit, then back off as you heard the motor whine become more prominent and really get things going. As speed increased the engine would rev more and more.

Now, I hear the engine rev grow in a much more linear fashion from the moment it starts. It isn't revving like crazy or anything, I just no longer hear that moment where it backs off and you really hear the motor whine as the most prominent sound.

My mileage has not been affected (~54MPG in non-winter). And while it seems a little less peppy than it used to, nothing else seems different.

My wife drives an '06 with about 30k on it. Hers sounds just like mine used to and while the her braking has always been smoother (I think due to the model year improvements), her acceleration has always been the same as mine. As I said, I do feel like mine has less pep that it used to.

Any ideas? Anything I should worry about?