Whistle at idle - vacuum leak, induction side

[RichieRoo]

Hi all!

Our little Yaris is a '99 1litre.

For the past few weeks it has developed a whistle when the car
is stationary and idling. The whistle disappears at higher revs.

The car idles a little slow and not perfectly smoothly - indicating
an air leak on the induction side.

My problem is that I can't locate the leak. I've checked all the
vacuum pipes whilst the engine is running and pushing/pulling
the plastic inlet manifold and throttle body doesn't alter the pitch
of the whistle, let alone stop it.

Does anyone know where they 'normally' leak?

Any advice warmly received.

Cheers

Rich