Idle noise, diagnose? (recording including)

[blehman]

Hi. This is my first post so want to begin by giving a BIG thanks to the e30 community for making this an amazing resource for a newbie. I've enjoyed seeing and reading about your projects!

I'm a new super e owner and unsure what is causing this noise when I idle. The sound is like rocks being tossed around inside a metal container, that's a poor mechanical description but for lack of better terms that's really what it sounds like. The noise is best heard under the car near the rear of the engine. The car idles at about 500-600rpm (normal?). I'm filling 'er up with regular 87 octane gas, and when first purchased ran a bottle of injector cleaner through. Exhaust is brand new from headers back and securely attached, so pretty certain it's not rattling.

Here is a recording of it idling:

http://drop.io/blehman/asset/bmw325e-idle-noise-mp3

Other than that noise the car runs pretty well for an almost 30 yr old car.

Any ideas?

[bernardsam]

Sounds like the exhaust to me mine sounds similar, Im missing the bracket that connects to the gearbox tho.

[sillybollox]

The clonking noise sounds like clutch release bearing to me. Does it do it if the pedal is down?

[choppa7]

Sounds like the exhaust to me mine sounds similar, Im missing the bracket that connects to the gearbox tho.

Just replaced mine having had the same problem although slightly different symptoms:

It was fine at idle, but increase the revs either driving or at idle to between 2k-2.5k and it would sound like a skeleton having a tug in a biscuit tin. I had a look under the car and couldnt see any place which would cause the rattle, it was only once I braced the exhaust with a metal bar to simulate the bracket I realised what it was.

Initial fears were that something had come loose in the gearbox due to the locatrion.

[JackCornick]

Mine makes a similar noise only more consistent. Im convinced its something like a dodgy bearing on the radiator fan, so its not spinning true. Iv got some play in my fan, i can wobble it around when touching it.

[Eddie25]

If it is the clonking noise you are referring to and it dissappears when the clutch is pressed I am told this is quite normal for an E30 mate.

It is to do with the bearings on the lay shaft IIRC and they did it pretty much from new. I thought mine was the release bearing at 1st as well but others have replaced it and the noise is still there.

This will need confiming by the likes of Brian Moore but I am pretty sure this is the case.

[blehman]

Hey, thanks for all the replies.

It is not the clutch, the noise remains constant when clutch is pressed in.

Having just replaced my Motronic (old one had a bad idle circuit) I paid closer attention to the symptoms today. Here's some more observations... gets better but does not go away when rpm's increased to 1000, it is inconsistent and occurs less when the car is warm, it is not idling smooth (the engine shakes a bit more than it should), and last there seems to be a miss fire when throttling up - acceleration is not smooth.

I am certain I have a good and correct Motronic unit, as well as new idle air valve, no vacuum leaks and mass air flow functioning correctly.

[blehman]

I will take another look at the exhaust... but pretty certain that it is secure being that it is all new and examined back in the spring.

[GeoffBob]

blehman, read this here

I'm inclined to agree with the others that the noise is coming from the lay-shaft (also called the counter-shaft, as I more commonly know it) in your gearbox . This is a symptom of poked bearings I am afraid.

What you have to keep in mind is that, when your gearbox is in neutral, the spigot (input) shaft (that begins at the centre of the clutch) engages the lay shaft via the 4th gear pair, but the lay shaft engages no other gears. The lay shaft thus spins freely and, if the bearings are poked, will rattle rather like stones in a biscuit tin. However, once you engage a gear and put the engine under load (drive away) the play will be taken up and the noise will stop.

While it is true that most RWD gearboxes do this (to a degree) from new, a severe noise (as in your case) is a definite sign of deteriorating bearings (which is a common fault on these boxes I might add). On the bright side, I heard exactly the same noise as yours on an old Alpha Romeo, and it still runs just fine .

Regards
Geoff