Thanks folks for your comments on what turned out to be a non-issue. Engine and exhaust system all check out fine, and (according to my service garage) the engine noise is normal... I am just driving the car harder now so I didn't notice the true sound of the car initially. Apparently I have one of the few Crossfires in Ottawa. It's not exactly the most appropriate car given our potentially severe winter conditions! I like that it's unique though -- turns a lot of heads!