I 've had several rattles on my 147. The rear catalytic convertor went, this gave a very noticeable rattle especially at slower speeds, tickover & bumpy roads - on open roads it was fine.
You live quite near me - Fareham Tyres on Gosport Road sorted my Cat out for about £ 180 I think. I expect they'll pop your car on the ramps to have a look.
As far as Alfa dealers are concerned - you have 4 local options:-
1) Perrys - lost the Alfa franchise about a year ago. Expensive & not very good service or local reputation for Alfas but very attractive receptionist!!
2) Meridien Milano - Havant Rd.,Drayton, Portsmouth. Took the Alfa franchise about a year ago. Rumoured to be very good & not over expensive. I've never used them (yet) except to buy a part. I was impressed with what I saw.
3) A & M Alfas, A32 North of Wickham (just before Roebuck Pub). Have used him a number of times. Small independant, small workshop but knows his stuff & will save you a fortune over an Alfa dealer. Downside - don't think he takes credit cards yet. But I've used him several times & will go back.
4) Southern Alfas, Shamblehurst Lane North, Hedge End - another independant been going several years. I'm told they're good but I've never tried them.
Hope you sort your problem out