Where I am, this is generally pronouced with a hissing 'h' made at the roof of the mouth. It's hard to explain. Try going to say 'nigg' and then making a sound that is a cross between 'ah' and 'ow' and something else as you curl your top lip slightly upwards. The end sound should resemble a cross between: drinking something cold (ah! as in That's better, I was thirsty) that catches you ('ow' as in you drank it too quickly and it gave you brain freeze) and a kind of 'I bit my tongue' noise.
I think that's the pronunaction. Of course, the accent could just **** it up a bit.
Or I wasn't thinking about it too much.