The mechanic who did the top and changed the cambelt at the same time said something like "I adjusted the timing a touch as I thought it idled smoother that way" Not sure if he advanced or retarded it via the cams or by some other way. However the knock was there before the head gasket and cambelt was changed.
If anything the engine is a bit sluggish at low revs, like under 2000 rpm. The power really kicks in at 3000 but that's typical of the TS.
I will use the stethoscope on the weekend and update when I check the cam timing.
I think that it is significant that your mechanic may have altered the 'timing' to improve the tickover!
There must be a problem for him to have chosen to do this. Did he check the cylinder compressions?
By the way, did you use an Alfa dealer or a local garage to do the work?
If a local did he have the equipment to set up the cam timing correctly?
I recollect that my twin spark was smooth on idle so check your exhaust (at the manufacturers idle speed there should be even and distinct pulses at the tailpipe)
This is quite relevant because, although you confirm that the engine has knocked since you bought it, it seems now that the above may be another symptom of the fault.
By the way, 2 oil changes in 9 months? Was the oil dirty when you changed it yourself??
Best wishes with your investigations this week end. You will need cam clamps to check the timing properly.