Any ideas on this one?

MK3 2.5 V6 Mondeo (same engine as the ST24, Probe, Cougar 2.5 V6)

With the bonnet open today to check over the usual's I thought I would start her up and listen to the engine.....I have a lot of time on my hands at the moment

I could hear a very faint induction kind of noise. You know when you have an induction kit and press the accelerator hard it makes a kind of suction noise, well my car is making this noise but very very faint. A blip of the throttle and it gets a bit louder for all of 1 second, just like an induction kit.....But I dont have an induction kit!

It sounds like its coming from the upper inlet manifold. Is there anyway to test for leaks or whatever this noise could be?