This sounds like the anti roll bar D bushes to me mate i'm afraid.
and the noise can occur randomly as you describe, as just following a slight camber in road is enough to either move the anti roll bar sidways against the bush stops, or knock it vertically within the play that has come from the gap found in worn rubbers.
some good old fashioned rough / bumpy roads and the car can be quiet as a mouse yes?
and this is because whilst the rest of the suspension does its job of moving vertcially to track the rise and fall in the surface, the anti roll bar is actually static, and doesn't need to move at all, but give it a one sided lean, or bumps on just one side, normally the nearside makes it more notable, and they can rattle their little bump stops off!
does that sound about like your car?
if it does, then welcome to my world!!!
i recently bought the same car, an 02 plate 2.0l lusso 147, and upon hearing the dreaded knocks and creaks that are so common with these 147/156 series cars, i ordered a full set of upper and lower wishbones and ball joints, and anti roll drop links.
if i'm honest, over the years, i have bought and fitted many many sets, they are all gits to do, real PITA's in my opinion, and the cheap one are no worse than the OE in my opinion, which fail pretty ruddy quickly if you ask me, so i don't believe anyone who says that if you buy Alfa parts, it would be better, because there are many many stories of the famed wishbone noises as early as 30k'ish miles, so why would i rush out and buy the same again?
sorry, just my experience, and could easily be wrong, but thats how i've found it myself.
anyway, a day of curseing and all the new parts are in, yippee!
test drive, still a noise, back on the ramps, everything is settled where it should be, everything tight, and the only part i didn't repalce is the D bushes, mainly because they are such a pig to do.
closer inspection, which in itself is tough becuase there is little in the way of a line of sight, there is a very small bit of visible play and wear, which also hardens the rubber making the noise sound worse than it is, and so i'm pretty convinced that is what i'm left with doing.
but i'm going to leave mine through winter, do it in spring if i still have the car, its not life threatening or anything, the car wouldn't be a deathtrap without the arb at all, and nothing can fall off!
some folk approach it by dropping the subframe all the way off rather than just lowering it sufficiently the feed the bar out, and some folk go for just new bushes and some favour a new bar complete.
the D bushes are held in by a saddle, but it can be tricky, its not as straight forward as it looks on a diagram, and room is Tight!!!
What do you think?
does this sound like your cars ailments too!