As mentioned previously, I have a 2004 WRX wagon with a stainless Prodrive circular-tip axle back installed around 2006. Lately, the weather has been nice enough to drive with the windows open and I've noticed a change in the exhaust note. There's a tight rev band where I notice a howl, especially when descending through 3200 rpms on a dropped throttle. This could be a resonance of some sort, since it is also noticeable (thought not as annoying) when maintaining steady revs.
My fundamental question is, however, what changed? I've had years with this exhaust (both axle-back and OEM). Am I hearing a change internal to the muffler? Or is it more likely that one of the other pieces is starting to fail?