Forester intermittent engine noise

[TheSeeker]

Hi,

I have a 2002 Forester with the 2.5L engine. Recently it has developed
an intermittent noise under the hood that can best be described like a
rattlesnake rattle.

This noise only lasts 3-5 seconds, and happens during an increase to
light throttle, usually around 2500 RPM, at 40-50 MPH. The sound seems
to start more quietly, but then will stop abruptly. It doesn't make
the sound idling, or coasting.

The character of the sound is like air whistling through an orifice,
but being valved on and off quite rapidly (5-10 Hz?).

Has anyone here ever had a sound like this? What could cause it and/or
how do I diagnose this further? I have not checked for engine codes
yet, but there is no CEL.

Thanks,
Duane

 

[1 Lucky Texan]

Hi,

I have a 2002 Forester with the 2.5L engine. Recently it has developed
an intermittent noise under the hood that can best be described like a
rattlesnake rattle.

This noise only lasts 3-5 seconds, and happens during an increase to
light throttle, usually around 2500 RPM, at 40-50 MPH. The sound seems
to start more quietly, but then will stop abruptly. It doesn't make
the sound idling, or coasting.

The character of the sound is like air whistling through an orifice,
but being valved on and off quite rapidly (5-10 Hz?).

Has anyone here ever had a sound like this? What could cause it and/or
how do I diagnose this further? I have not checked for engine codes
yet, but there is no CEL.

Thanks,
Duane

One frequent suspect for any buzzing noise is exhaust heat shields.

 

[.._..]

Heat shields. Sounds like some kid took a vacume motor, clamped a stick to
it and is beating an empty metal can right? Your car is about the age where
that starts. Don't remove them, just get a bunch of those band clamps (the
kind with the slots on the band and the screw to tighten), poke at them
(when they are cold!) and apply as necessary. My car has 6 already, and
needs to go in for more. Several other Subie owners I know have had the
exact same problem.

Some people say stuffing steel-wool under them works too. But I would think
that would rust a lot faster than a bend clamp does.

Count on re-applying them every few years as they will fall off.

The heat sheilds do two things; 1) help prevent accidentally starting fires
by parking on or driving over grass and such 2) keeps the exhaust pipe hot
so the catalytic converter can work properly, if you remove them, you run
the risk of your ECU going wonky or throwing errors (check engine light) due
to the sensors picking up the change. It could also affect your gas milage.

In any case, take it to one of the dumbass muffler places with a bunch of
the clamps and have them do it, or if you can get under it, you can do it
yourself.

You could also replace all the shields, but that may get more expensive than
it's worth. It's just a noise, it doesn't hurt anything.

 

[DuaneKaufman]

Hi,

I have a 2002 Forester with the 2.5L engine. Recently it has developed
an intermittent noise under the hood that can best be described like a
rattlesnake rattle.

This noise only lasts 3-5 seconds, and happens during an increase to
light throttle, usually around 2500 RPM, at 40-50 MPH. The sound seems
to start more quietly, but then will stop abruptly. It doesn't make
the sound idling, or coasting.

The character of the sound is like air whistling through an orifice,
but being valved on and off quite rapidly (5-10 Hz?).

Has anyone here ever had a sound like this? What could cause it and/or
how do I diagnose this further? I have not checked for engine codes
yet, but there is no CEL.

Thanks,
Duane

Hi,

Thanks for the suggestions. I have had a number of heat shields make
noise, and have tracked them down. This doesn't sound at all like
them, and has a different characteristic on and off pattern.

I paid more attention to it this morning on the drive to work, and it
starts _always_ at 2300 RPM (no matter which gear, but must be at
light throttle) and stops before the engine reaches 2500 RPM. With
level or slight upgrade to the road, I can make it come and go at
will, for as long as I hold it at those RPMs.

In terms of what it sounds like, it sounds like an exhaust manifold
leak, but they usually make noise at all RPMs (especially at low RPMs)

Is there anything in the emission control sub-system that opens and
closes, or the evaporative emissions system?

Thanks,
Duane

 

[.._..]

Here's a link to an image with two examples of what they look like:

http://www.fftec.com/Tech/1/33.jpg

On the left the silver-y formed part with the empty screw holes is one (that
protects the car), and on the right the silver-y part that says "Subaru" on
it with the flange with the big bolts in it (above the white filter). That
is screwed on around the actual exhust pipe.

So the noise is one of these things rusting a bit and flapping when the
exhaust vibrates with a certain amplitude and frequency.

You just gotta find the place and wedge something in there, use a clamp of
some kind, replace the shields, or the exhaust part. It's all a matter of
getting in and seeing what moves that shouldn't, look ofr corrosion.

 

[1 Lucky Texan]

Hi,

Thanks for the suggestions. I have had a number of heat shields make
noise, and have tracked them down. This doesn't sound at all like
them, and has a different characteristic on and off pattern.

I paid more attention to it this morning on the drive to work, and it
starts _always_ at 2300 RPM (no matter which gear, but must be at
light throttle) and stops before the engine reaches 2500 RPM. With
level or slight upgrade to the road, I can make it come and go at
will, for as long as I hold it at those RPMs.

In terms of what it sounds like, it sounds like an exhaust manifold
leak, but they usually make noise at all RPMs (especially at low RPMs)

Is there anything in the emission control sub-system that opens and
closes, or the evaporative emissions system?

Thanks,
Duane

Any service done recently? I have read of instances where a whoosing
noise is heard if the 'tabs' on the lower part of an airfilter housing
do not get 'hooked' correctly.

Another possibility for a thump/knock could be timing belt rubbing due
to weak tensioner. But that doesn't seam like what you describe. Can
somone rev the engine for you to help pin down the location?

 

[DuaneKaufman]

Any service done recently? I have read of instances where a whoosing
noise is heard if the 'tabs' on the lower part of an airfilter housing
do not get 'hooked' correctly.

Another possibility for a thump/knock could be timing belt rubbing due
to weak tensioner. But that doesn't seam like what you describe. Can
somone rev the engine for you to help pin down the location?

Hi,

No Service done lately.

When the car isn't moving, the sound doesn't show up.

Thanks,
Duane

 

[1 Lucky Texan]

Hi,

No Service done lately.

When the car isn't moving, the sound doesn't show up.

Thanks,
Duane

Maybe a split CVJ boot has a 'flap' or something on it?
Is there any 'jerking' or vibration? Any clicking or other problems
when turning? maneuvering at low speeds? Are thires in good condition?

 

[DuaneKaufman]

Maybe a split CVJ boot has a 'flap' or something on it?
Is there any 'jerking' or vibration? Any clicking or other problems
when turning? maneuvering at low speeds? Are thires in good condition?

Funny you should mention a CV boot, for as I was looking around the
engine compartment for things that could be making noise, I noticed
that I had indeed developed a ripped inner front passenger-side boot.

For that, I took it in to a dealer to have the axle replaced, and
asked them to look into the rattling too. They said they could not
reproduce it, but replaced some anchors for the foam on the lower
engine cover.

No vibration
no clicking when turning
tires all look good

The sound seems to be tied to engine RPM like clock-work. I can get it
in any gear at 2300-2500 RPM.

The sound is still there, so I think I'll have to look into it more
myself.

Thanks,
Duane

 

[1 Lucky Texan]

Funny you should mention a CV boot, for as I was looking around the
engine compartment for things that could be making noise, I noticed
that I had indeed developed a ripped inner front passenger-side boot.

For that, I took it in to a dealer to have the axle replaced, and
asked them to look into the rattling too. They said they could not
reproduce it, but replaced some anchors for the foam on the lower
engine cover.

No vibration
no clicking when turning
tires all look good

The sound seems to be tied to engine RPM like clock-work. I can get it
in any gear at 2300-2500 RPM.

The sound is still there, so I think I'll have to look into it more
myself.

Thanks,
Duane

You may need to take a technician on a ride and create the sound for
him.

Other than that - i dunno. Hope it isn't expensive!

good luck and let us know what you discover.

Carl

 

[.._..]

Not to re-start this thread, but I had a noise similar to that when my head
gasket blew. Maybe check the coolant level, check for bubbles in it, and
inspection for coolant on the outside of the motor?