I've had the heat shield changed on my 147 1.6 ts twice now. The second time did not cure the buzzing at around 2000 rpm. Turns out that the supporting bracket between the engine block and manifold was cracked. Alfa Aid of Maidenhead reckon it should almost be changed at service time and promptly produced a bag full from under the counter. Dealer could not find the problem but agreed there was a noise!