Resonant noise when accelerating. Any guesses on the cause?

[Paul Rybarczyk]

Hi. I've got a 2000 Outback Legacy 4-cylinder 5-speed with 31,000 miles
that's approaching three years old. Last weekend I took it to my
mechanic (not a Subaru dealership) for a tune-up (new oil, plugs, and
new oil/fuel/air filters) . I'm not sure if that's related to this
problem, but pretty much the next day I noticed a loud low pitched
vibration/sound resonating through the cabin. It sounds like it's coming
from the exhaust system near the engine. It gets noticeable at about
2000RPM and just gets louder from there, but*only* when the engine is
under high load. By 3500, it's loud enough to drown out a normal
conversation or the usual sound of the engine. However, if I accelerate
more gradually (like I usually drive) everything sounds completely
normal up though 4000RPM.

My mechanic put it back up on the lift, but we couldn't find anything
wrong. The exhaust system was not touched as part of the tune-up, and
nothing seems to be rattling or loose. They also took a ride with me and
heard the noise, but couldn't figure out the root cause. They've
provided me excellent service in the past, but they are basically
stumped. Since they don't know what else to check, they suggested taking
it to Subaru.

To me, the noise sounds very much like the loud resonant vibration that
one of my previous cars made when the exhaust pipe detached from the
catylitic converter. But that was much louder and pretty loud all the
time. My Outback is *only* loud when floored above 2000RPM. Otherwise,
it runs/sounds fine, and power output seems normal.

So anyway, that leads to a few questions:

Any guesses about the source of the sound and why it's so dependent on
engine load?

Anything else my mechanic could check? They would probably do so for free.

Should I take the car to Subaru? I'm assuming this will not be convered
by my warrantee and I don't want to spend big $ to play guessing games.

Any possible harm in just living with it until the cause becomes more
obvious?

If you have any questions, please let me know. Thanks in advance.
--
Regards,
Paul Ryarczyk
paul at rybarczyk dot net.

[The Googster]

Do you also notice it while, say, idling at a stoplight on a cold morning?
If so it might be the heat shields around the exhaust system. I had the
offending parts removed by my local dealership when I had similar noises.

"Paul Rybarczyk" <pa...@REMOVETHISrybarczyk.net> wrote in message
news:3E80DF50...@REMOVETHISrybarczyk.net...

[Greg Truax]

I had a problem similar to this on my 2001 Forester. It would get really
loud in a certain rpm range. What happened was that when service was
performed on the car, they opened the "air box"(sorry im no mechanic, this
is as best as I can describe it) to check the air filter, and the box
wasn't put back together properly, there were some clips that are a bit
hard to align. I guess the service people in their haste didn't make sure
it was back together the right way, and fixing it was just a matter of
making sure the clips are locked down. In the forester this box is right
around the middle and up close to the windshield under the hood.

I hope for your sake this is the problem...its an easy instant fix, as
nothing is really broken.

Hope this helps
--greg

[dasa]

i, too, had a similar incident when the air filter housing (the black
plastic box casing the air filter, aka "air box" :-) was not closed tightly.
the engine sounded like those cars with megaphone muffler. i rather enjoyed
the engine sound initially, but guess i decided it wasn't worth choking the
engine with dirt that way. hope that sort out your problem.

dasa

"Greg Truax" <gk...@cornell.edu> wrote in message
news:b5qtrv$sko$1...@news01.cit.cornell.edu...

[romy]

It's almost a certainty that the air filter housing was not put back
properly. Make sure the top part of the housing is seated ok.

"Paul Rybarczyk" <pa...@REMOVETHISrybarczyk.net> wrote in message
news:3E80DF50...@REMOVETHISrybarczyk.net...

[Edward Hayes]

There are notches on the bottom of the filter box that must be inserted
first before the top can be seated. It's very easy to not do correctly. Ed

"Paul Rybarczyk" <pa...@REMOVETHISrybarczyk.net> wrote in message
news:3E80DF50...@REMOVETHISrybarczyk.net...

[Paul Rybarczyk]

This newsgroup is awesome!!! :-)

Well, as you probably all guessed that was it. The box wasn't
connected at the bottom at all, so my mechanic removed it and
re-assembled it correctly. Problem solved. I never would have
guessed it was the air filter box.

Thanks everyone for your help!
--
Paul
email: paul at rybarczyk dot not

[Warren O]

(Yes, this follow-up post is a bit late...)

I second that sentiment! I had the exact same problem...

I *thought* I was being a good boy by getting an oil change at Iffy Lube right
after the Baja's 1000 mile break-in period. When I left, I thought it sounded
louder, but I wasn't entirely sure since I hadn't really 'stepped on it' much
during the break-in. I brought it back the same day, and they couldn't find
anything wrong, other than the extra half quart of oil they put in it. :/

Nearly 2000 miles later, I was pretty sure something had to be amiss, perhaps
with the exhaust. I was planning to have the dealer look at it when I brought
it up to be checked out in another 1000 miles or so, but then... I read this
thread.

Sure enough, they had done me the 'favor' of checking the air filter, but
didn't close the bottom correctly. Fixed that, and now it is smooth and quiet
once again, so I am zooming about happily. :)

Is there anything I should have checked due to my air filter being messed up
for such a long time?

Moral of the story: NEVER go to a quick-lube shop during the lunchtime rush.

-Warren.

On Sat, 29 Mar 2003 15:41:51 GMT, Paul Rybarczyk <pa...@REMOVETHISrybarczyk.net>
wrote: