Had the same on my 2.4JTD, changed all the usual stuff aux belt, pulley, tensioners etc all in cost £380.00, still had the noise although a little less noisey and seemed to go when the weather was damp.
Then, whilst moving a around at 5mph there was a load bang and a horrible expensive sounding noise.
Looking in the engine bay expecting the worst I saw a pulley resting against the inner wing.
It was the Alternator pulley which has a clutch type mechanisum in it, the pulley was replaced (£80.00) and now now more noise, so if you have replaced most things that you think is causing the noise check the alternator pulley.
PS Thanks to Jamie at The Alfaworkshop for sorting the problem out and Greenflag for recovering me in less than 20mins even though I was in a farm yard, on a Sunday, down a lane in the middle of nowhere near Fingringhoe, Essex.