I read this on http://www.alfa156.net/
a while ago and sort of sound like your problem,
Particularly in cold weather and driving over speed bumps
you should watch out for squeaking from the front suspension.
This squeaking is NOT normal, but a common fault on Alfa 156.
This might have been corrected on later models, though!
It can take as little as two months from new to they make this
squeaking noise. The noise is coming from the upper front
wishbone bushes and can be fixed by greasing or just changing the
whole wishbones. The parts are not expensive and there should be
better ones available.
One tip is to check the lower wishbones. They are a bit expensive but it's
possible to squirt a little oil with a medic needle inside the rubber that
covers the wishbones connection point.