Saxo 1.1i Squeak when I ease off the gas?

My car has started making a strange squeaky sound when I ease off the accelerator, and sometimes when i'm holding the pedal steady. I've took it to 2 garages and had it serviced and they can't work out where the noise is coming from but say that nothing is drastically wrong with the engine. Its unbearable to drive now, as it's so noisy. Its a strange noise and a good way to describe it is to imagine the sound a pigeon makes..!? Sounds like something needs a good oiling. Anyone got any ideas as I don't know what to do next