Odd Engine Noise

[ScottD3]

I'm going to do my best to describe this but I feel I will fail miserably and due to the nature of the noise I doubt I could film it.
I'll do my best to make use of the English language though.

The car:
A4 98 1.9TDI 110AFN Auto. 157k, 3k miles away from a service.

Symptoms:
Between 2.5k RPM and 3k RPM fast moving air with added sand kind of sound is heard.

VCDS:
No Codes.

I noticed it about a week or so ago and been trying to work out what it is. Its not a noise I've ever heard before. First I thought it was air cause by a pipe popping off but all pipes seem ok.
A couple of times I've stopped, put the car in N in an attempt to hear the noise and there is nothing. It just sounds like a diesel engine revving at about 3k RPM.
The sound only happens when I'm travelling and speed. Which gear I'm in is not a factor that I'm aware of, as long as I'm moving. I've tried it in 2nd and 3rd doing different speeds and it still happens.
With the revs between 2.5k and 3k, the noise woooooshing away merrily as I travel down the road I do a couple of different things in a hope to hear it from the outside.
I open one or both of my front windows as I'm driving past walls, fences, hedges or poeple, pretty much any thing in the hope the sound will bounce off and give me a better idea of what the noise is but it don't help. I just hear wind or the diesel engine ticking away.
The sound fades in really quickly and out just as fast that I thought it was on and off not fading but more listening to it and its defiantly fading in and out.
My next attempt to hear the sound might involve some car park hooning, a mate under the bonnet or possibly a camera strapped somewhere.

That's it I think. I'm out of ideas.

Any one got any ideas?

 

[Blue_Thunder]

Is it happening on boost, off boost or both?

 

[ScottD3]

Is it happening on boost, off boost or both?

I'm assuming its boost cause of the revs when it happens at.

Does boost still happens when you're stationary?

 

[AlexGSi2000]

I'm assuming its boost cause of the revs when it happens at.

Does boost still happens when you're stationary?

Not really - the engine needs to be under load for the ECU to tell the actuator to make boost.
You generally hit boost when you are moving and put your foot down, if your carrying a fairly consistant speed you wont be boosting much at all.

Sounds like it could possibly be a pulley at the front of the engine? - can you get a sound recording?

 

[ScottD3]

I can try and get one but its hard since I got to be boosting.

It stops soon as I take my foot on the throttle.
Some more playing and it could be done on boost only.
Turbo on its way out?

If its a pulley on the engine, it would always make the noise, boost or no boost right?

 

[AlexGSi2000]

Could possibly be a boost leak then, check all your boost and vacuum pipework for small splits.

 

[Oranoco]

Check underside of top rubber boost hose. Mates Passat made the described noise shortly before this pipe split.

 

[AdamL]

Mine did this earlier last year, turned out as above top rubber coupling onto inlet manifold had split on the underside next to one of the jubilee clips. Glaringly obvious when found but again, could not replicate free revving the engine with no boost being produced. Sounded like the turbo was trying to produce 40 pounds of boost, couldn't have done it much good though. Hope this helps

 

[ScottD3]

Thanks for the hints lads.

The more I read the above and think about it the more it sounds like its boost related.

I had 30-45 minutes of a light left yesterday after work so I popped out and had a quick look and poke of the pipes.
Could not see any thing obvious but I will have a better look tomorrow, take them off and give them a clean at the same time.

I took off the rubber pipe that joins the long plastic pipe, I got no idea on names BTW, reconnected it and the car seemed quieter on the way in to work this morning but then it came back.

I'll check the others tomorrow and report back.

 

[aragorn]

Could be the intercooler itself as well. The end tanks can split.

 

[ScottD3]

Its going to end up a bumper off, up on ramps style of hunt?

 

[ScottD3]

Dropped by Phil Miller's garage to use his smoke tester.
Setting up the smoke tester and we found the split pipe.
The short rubber one that's on the inlet.
Replaced it Wil s silicon one and it works a treat now.


Thanks for the info lads.