Whistling Noise

[Tinker15]

I've just been out for a very pleasant drive to Buxton in Derbyshire but once the engine was warm I developed quite a loud whistling sound which appears to be coming from the engine bay. It doesn't appear to be much affected by engine revs or road speed but interestingly disappears when applying throttle. However, once you take your foot off the accelerator the whistling noise returns immediately and is there even at low speed. Once back on my drive I couldn't hear any noise with the engine ticking over or if I revved it.

All very strange. Any suggestions as to where I should start looking would be much appreciated.

[selnic]

Have a good look around the stepper motor housing area. I have seen what appears to be the same problem as you describe, turned out to be the rear gasket had developed an air leak which whistled when the throttle was closed. You can find the leak by spraying easy start onto the flange with the enging running, if the gasket has gone the easy start will be sucked in. HTH

[slideways]

Does it sound like this?
http://www.youtube.com/watch?v=Z0g00Wx7mi8

[LukeSi]

slideways said:

Does it sound like this?
http://www.youtube.com/watch?v=Z0g00Wx7mi8

That sounds amazing.

[Tinker15]

selnic said:

Have a good look around the stepper motor housing area. I have seen what appears to be the same problem as you describe, turned out to be the rear gasket had developed an air leak which whistled when the throttle was closed. You can find the leak by spraying easy start onto the flange with the enging running, if the gasket has gone the easy start will be sucked in. HTH

Many thanks. Will check that tomorrow.

[Tinker15]

slideways said:

Does it sound like this?
http://www.youtube.com/watch?v=Z0g00Wx7mi8

Regrettably not! ;-)

[Dr Mike Oxgreen]

Yep, definitely sounds like an induction leak. The clincher is the fact that it disappears when you open the throttle. With the throttle closed, the engine is sucking hard against that closed butterfly - so the vacuum in the induction system is greater. Any leak is going to show itself most under those conditions. The effect will be at its greatest with the engine at higher revs with the throttle closed - like when you've just lifted off for a bend. Open the throttle, and the induction vacuum reduces because there's less restriction to the influx of air.

Edited to add: Try reading the following article. It's aimed at pilots of piston engined aircraft, but it's highly relevant to any normally aspirated piston engine. It explores the relationship between induction manifold pressure, RPM, and throttle position (oh, and altitude).

Manifold Pressure Sucks!

Edited by Dr Mike Oxgreen on Sunday 7th October 19:54

[carsy]

Sometimes the base idle screw on the plenum can cause a whine type noise. Small turn one way or the other will often get rid of it.

[davep]

It's probably audible Helmholtz plenum/inlet resonance and as carsy said tweaking base idle gets rid if it's annoying.

[Tinker15]

Thanks to all those who responded to my query.

I've checked the gasket which appears to be OK. I've given the idle screw a turn and tightened up the clips on the tube from plenum to AFM which were quite slack. A blast over Saddleworth Moor produced a faint noise at first but this disappeared in a few minutes. On inspection the induction hose elbow doesn't look that good so I've ordered a smooth bore version from ACT. I'm hoping this will be the end of my whistling problem.

[Chilliman]

Tinker15 said:

Thanks to all those who responded to my query.

I've checked the gasket which appears to be OK. I've given the idle screw a turn and tightened up the clips on the tube from plenum to AFM which were quite slack. A blast over Saddleworth Moor produced a faint noise at first but this disappeared in a few minutes. On inspection the induction hose elbow doesn't look that good so I've ordered a smooth bore version from ACT. I'm hoping this will be the end of my whistling problem.

[Bassfiend]

LukeSi said:

slideways said:

Does it sound like this?
http://www.youtube.com/watch?v=Z0g00Wx7mi8

That sounds amazing.

Oh God I want that soooooo badly!