I've got exactly the same problem on my 147. It's driving me nuts. I've read a lot about it possibly being wishbones or bushes that need replacing. I'm reasonably sure my creaking noise is not suspension related though. Although it creeks when I pull-off and go over bumps, the creeking disappears when the engine is off despite me jumping up and down inside the car! (The old techniques are always the best). I'm starting to think there's a lose bracket around the front of the exhaust or around the engine if there could be such a thing - although I've really not got a clue.

Any thoughts welcome before I go and take the plunge at an Alfa garage.