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EDIT: Additional information provided by LovelyWife(TM) - listening outside the car, she has proclaimed that the rattle / buzz / drone / resonance is coming from the front of the car and sounds like it's coming from below the engine between the wheels.

I was overly optimistic in my assessment of the previous question: How should I diagnose an exhaust "howl" when descending through 3200 rpms?

Here is the current situation on my currently all-stock (intake and exhaust tracts) 2004 Subaru Impreza WRX: when revving carefully around 3250 rpms (+/- 100 rpms), there is a distinct jump in the overall noise level on a totally different peak frequency (higher by about an octave on my uncalibrated ears) from the standard exhaust note. This is a super-annoying drone that is easily sustained and right at a convenient cruising speed.

This isn't present (or is greatly reduced) under acceleration but is very obvious at a steady speed or when descending through that rev band. This induces me to think that the problem is downstream of the turbo as it's most obvious in an off-boost situation.

Here's what I know so far:

  1. It's not the stock muffler - the problem was much more obvious but still present in the aftermarket muffler.
  2. It's not the aftermarket muffler - see 1.
  3. It's not the gasket on the muffler - replaced.
  4. It's not the nearby heat shielding - same problem with and without the shielding.
  5. It's very hard to tell if the problem exists with the muffler removed entirely as it's swamped but all the other noise (or song, depending on your preference).
  6. It doesn't appear to be the donut ring on the intermediate pipe after visual inspection.
  7. There don't appear to be any obvious holes or leaks in the exhaust tract downstream of the turbo after crawling around under the car all morning.

So, questions:

  1. Am I hearing the resonator on the intermediate pipe fail?
  2. Am I hearing the pre-cat on the uppipe fail? This is a fabled bogeyman since the WRX was first introduced to the US.
  3. If there really is a hidden exhaust leak, why would it be so tightly bound to a single rev range?
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Does the excessive noise occur when you are revving the engine without the transmission engaged? If so I would take the car to a shop with a lift, have someone rev the engine to the annoying range and then start looking on the underside of the car to pinpoint the noise. –  Patrick Jul 2 '11 at 2:08
    
@Patrick, yes, exactly - the sound is reproducible with and without load. WRT taking the car to the shop, I'm trying to avoid that just as a convenience issue. –  Bob Cross Jul 2 '11 at 4:04
    
I believe the reason the sound is tied to a specific RPM range is that the vibration the engine is producing is close enough to the vibrating piece's resonant frequencies that the amplitude of that frequency increases a great deal. More info at Wikipedia. –  Patrick Jul 2 '11 at 5:00
    
@Patrick, yes, that was my first thought. However, my experience is primarily focused to things in water so estimating based on a speed of sound of 1500 m/s isn't helping me here. Going to try the close-up inspection method. –  Bob Cross Jul 2 '11 at 14:19
    
@Patrick, you were right: see the update below: mechanics.stackexchange.com/questions/1276/… –  Bob Cross Jul 30 '11 at 18:20
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1 Answer

up vote 1 down vote accepted

A trip to the dealer had the car up on a lift with a puzzled mechanic wandering around underneath, repeatedly causing the rattle to trigger. After a while, we both looked under the car and have concluded that the problem is the heatshield over the right hand exhaust manifold.

That isn't terribly surprising: heatshield metal has never been good and, after 114000 miles, it hasn't improved.

His conclusion was to leave it alone: his labor would have been cost prohibitive just to replace the shield: you can't just cut the bolts and pull the shield loose as that's where the front O2 sensor goes. You'd have to cut a notch in the shield or pull the O2 sensor (no thanks).

So, at this point I'm trying to think of ways to better secure the shield in place or generally break up the harmonics at that rev range.

Yet another update: I managed to get a very large hose clamp (metal ring with a tightening screw) all the away around the questionable heat shield. I tightened that down and clipped off the remaining tab of clamp and the current result = no unpleasant noise (as yet).

In the future, I will likely replace the downpipe and, in the process, take off that shield for replacement and / or adjustment.

EDIT: after a little more research, that's actually an engine gas temperature sensor (not O2) that's effectively monitoring the state of the pre-cat that's in the up-pipe. If it turns out that the problem is not the heat shield but is, rather, the up-pipe itself, I may be a lot more interested in taking that apart.

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