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?