"throaty" engine noise?

[spursfan]

I'm sure there is a thread somewhere that mentions this, but I have searched around and can't find it. I loaned my 04 9-3 linear (34k mi) to a friend for a weekend a few months ago, and when I got it back, all of a sudden the engine sounds completely different than before. For lack of a better description, it sounds much more muscular or throaty than it ever did - like an american muscle car as opposed to - well - a Saab. It doesn't happen all the time - maybe every other trip, and it doesn't seem to affect performance. But it is REALLY annoying and takes some of the fun out of driving it.

Anyone have any idea what might be causing this? I thought maybe my friend put lower octane gas in it, but that would have flowed through long ago.