Firstly, I've got a 1998 white Alfa 156 1.8ts.
I've recently replaced the whole exhaust system except manifold, managed to do it all relatively cheaply. But unfortunately I may have inadvertently caused myself a problem.
The car is making a metallic vibrating noise through the rev range, but worse after 3500 revs. It sounds to be coming from the exhaust. When I'm under the car it sounds like the back box. I've since taken the backbox back and had two replacements, all were slightly different and from different manufactures. The sound remains though!
I've checked all the heat shields, which have good clearance. I've taken the flexi pipe off again to check for excess flex inside the pipe as per: http://www.alfaowner.com/Forum/showthread.php?t=164698 (Unidentifiable exhaust noise)
All seems okay within here.
I know my MAF is dodgy, so was thinking could it be pinking? I've disconnected the MAF - The car seems very lumpy and nearly stalled (I don't remember it being like that when I replaced the MAF last time - Yes it was a cheap ebay buy who promised the part was genuine, should have known it wouldn't have been!!)
But the noise remains. So I checked all the spark plugs, which have a nice tanned colour to them, not to dark or to pale. Which suggests to me there's no over/under fueling.
I'm all out of ideas, can anyone offer any advice? Any further checks I can do, or people with same issue?