Steady whistling in cabin during light/off load?

[Drecian]

So this morning my AU1 Fairlane started a quiet whistling inside on light load. It only happens in cruise or off throttle, or basically high vacuum, but is not RPM dependent (same sound off throttle at any RPM). I tip in to load up the engine and it goes away. The noise doesn't really vary in pitch or volume either, it's there or it's not. It's almost like radio interference noise, but the audio system is stock and it doesn't rise with RPM like interference usually does.

I opened the bonnet thinking it might be a vacuum leak, but couldn't hear anything out of the ordinary in from the engine bay, and definitely not the usually hissing from a vacuum leak.

As for how loud, it's just audible with the windows up and radio off. Crack a window open and the wind noise it louder.

No running issues, idles and accelerates fine on both LPG and petrol (thinking it may be LPG related, but whistles on both fuels).

Noise stays the same whether the blower fan is MAX or OFF.

Could it be a vacuum hose off a heater box actuator? Having played with this (not on this car) before, the hoses were pretty tight and I couldn't see one just vibrating off.

Anyone?

[mif73]

Originally Posted by Drecian

So this morning my AU1 Fairlane started a quiet whistling inside on light load. It only happens in cruise or off throttle, or basically high vacuum, but is not RPM dependent (same sound off throttle at any RPM). I tip in to load up the engine and it goes away. The noise doesn't really vary in pitch or volume either, it's there or it's not. It's almost like radio interference noise, but the audio system is stock and it doesn't rise with RPM like interference usually does.

I opened the bonnet thinking it might be a vacuum leak, but couldn't hear anything out of the ordinary in from the engine bay, and definitely not the usually hissing from a vacuum leak.

As for how loud, it's just audible with the windows up and radio off. Crack a window open and the wind noise it louder.

No running issues, idles and accelerates fine on both LPG and petrol (thinking it may be LPG related, but whistles on both fuels).

Noise stays the same whether the blower fan is MAX or OFF.

Could it be a vacuum hose off a heater box actuator? Having played with this (not on this car) before, the hoses were pretty tight and I couldn't see one just vibrating off.

Anyone?

Maybe vac leak in heater actuator assy or the Climate control sensor vacuum fitting partially blocked? (it sucks cabin air past the thermistor in the MDC to check cabin temp via engine vacuum).

[Drecian]

Originally Posted by mif73

Maybe vac leak in heater actuator assy or the Climate control sensor vacuum fitting partially blocked? (it sucks cabin air past the thermistor in the MDC to check cabin temp via engine vacuum).

So the noise stopped on the drive home from work that afternoon and hasn't come back since. Kinda half waiting for it to start again (but hoping it doesn't) so I can troubleshoot.

I was thinking it might be one of the vacuum motors in the heater assembly too, can't really think of any other vac sources inside.

I thought the CC temp sensor had a little fan to pull air from the sensor on AU's...?

[mif73]

Originally Posted by Drecian

So the noise stopped on the drive home from work that afternoon and hasn't come back since. Kinda half waiting for it to start again (but hoping it doesn't) so I can troubleshoot.

I was thinking it might be one of the vacuum motors in the heater assembly too, can't really think of any other vac sources inside.

I thought the CC temp sensor had a little fan to pull air from the sensor on AU's...?

Hi Jeff,
AU's definitely use vacuum to draw air past the sensor, if you pull out your climate control (MDC), you will see a vac hose connected to the back of the little vent part near the clock.
I think BA on use a fan motor like a lot of other cars.

Cheers,
Andrew.