3k rattle thread (with very specific symptoms)

[Steven103]

Hi all,


Would really appreciate any suggestions, been chasing this rattle for a while. Built up quite a specific list of exactly when it happens. Searched a lot and a checked all the very common bits.

Symptoms

* ONLY when you're on the throttle (not at a constant RPM or when backing off the throttle through the above rev range)
* REGARDLESS of throttle position (accellerating as gently as possible or full throttle, little difference)
* ONLY when engine is under load (i.e. doesn't happen on axle stands)
* ONLY happens when in a gear (doesn't do it with clutch down and with a gear selected though, so can't be the gear linkage hitting anything etc)
* Happens LESS going downhill (starting to sound engine-position related as it moves backwards on mounts when under load)
* Regardless of engine temperature
* 2700 ish RPM (1st & 2nd gear), 2800 RPM (3rd gear), 2900 RPM (4th & 5th gear)

It isn't:

* Cat > mani gasket
* Cat > backbox olive joint
* Rear heatshield
* Centre heatshield
* Exhaust mounts
* Blowing exhaust
* Knackered lambda sensors
* "The noise a decat makes" as per another current thread

I can't rule out:

* Manifold on heatshield (though there is probably 20mm clearance most of the way around as I can get my hand in there. Obviously I can't look at this whilst the car is being driven - the only time when it happens)
* Exhaust manifold gasket (saw this suggested in another thread)
* Gearbox/Engine mounts (they look in very good condition, but obviously can't check for movement whilst car is being driven)
* Catalyst (spoke to another member who's similar sounding. No way to verify this though without a standard decat pipe, unless I drive around with the cat temporarily missing entirely...)

Other points:

* Exhaust system vibrates with a surprisingly large magnitude at 2 to 3k RPM (not sure if normal? certainly can't be ideal). Oddly it also vibrates an impressively little amount at 3.5 to 4k (exhaust mounts tuned for motorway speeds in 5th gear etc, but even so)
* Fairly noisy idle, but smooth in terms of vibrations & no rev hunting
* Recent oil/filter/plugs/gbox oil, lambda, exhaust mounts for what it's worth
* The fact it doesn't happen when you hit bumps/potholes/speedbumps etc indicates that it isn't just something that is loose (i.e. the usual heatshields, ECU bracket, cat>mani joint)
* Bringing the clutch up with the handbrake on & 3k RPM (putting it under load) doesn't replicate the noise
* Been there for a few months, would've expected something to have fallen off by now if that were the case


Thanks in advance

 

[sinnie92]

Nice article, try the following:

Ecu bracket, its about 30-40 mm long and made from 1mm steel and is prone to fatigue and eventually cracks.

Alternator stay bar, a long aluminum support bracket that goes between the alternator and engine block, to diagnose this put your hand on it whilst an assistant replicates the noise. It's usually the block bolt that decides it no longer wants to be attached.

Grab the airbox and try to move it, the bracket underneath can go.

Other than that, check for fouling of exhaust at the rear of car. check anything that is easy to get to that could rattle loose basically, even the oil filler insert on the engine cover can resonate...

Edit, I didn't entirely read your post. I had a similar problem when driving, how straight is your olive joint on the cat, bent at all?

 

[Blacky950]

Check your fuel rail guard mine was loose and rattled like the bottom end had gone!

 

[Steven103]

Had already tried ECU & alternator bracket. Airbox mounts checked when checking gearbox mount.

Olive joint is securely and doesn't blow, but didn't pay particular attention to it's angle - have you had this cause load-only rattles in the past?

Fuel rail guard is absolutely solid. Thanks for the suggestion though!

 

[sinnie92]

Yes dude, I recently had a problem with the olive joint clamp vibrating against heat shield on the right hand side of the exhaust tunnel, in my case the catalytic converter was bent to the right and down, it was about 10mm away at the closest point with the engine off.

Are you 100% sure nothing blows? a whistle under load is usually a good indication of either manifold-cat or cat-centre joints.

I will give you some before and after picks of my cat which may help you. will be in a bit cause they are on phone...

 

[milez89]

Did you ever manage to resolve the rattle? Mine has identical symptoms and its driving me crazy....