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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?

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up vote 3 down vote accepted

Check the seals where the axle-back attaches to the stock exhaust, you may have leak which can change the pressure and thus the sound.

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you're right - that's the first thing that I should look at. I was also thinking about the sound deadening material in the muffler itself. –  Bob Cross Jun 9 '11 at 0:34
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you were right - I disconnected the muffler and was immediately showered with powderized gasket. A new gasket significantly reduced the "howl" - there seems to be a change in the exhaust sound material after all these years. BTW, noted a typo in the original - this is an axle-back section. –  Bob Cross Jun 13 '11 at 13:53
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