Saab: Tapping/knocking, acceleration and deceleration, change characteristics

Question
Just bought my car saab 9-3se 02. Unfortunatly the turbo blow on way home.
Since then replaced turbo and obvious oil etc, car was fine was told to run it
for a few days then get another oil change. Done about a 100miles no
problems no noise then got the oil done, put in semi synth 5-30, noticed
straight away a tapping noise from top of engine, this starts when cold and is
still there when warmed up. It stay pretty constant and dose not seem to get
louder or quicker. I told the man this and we changed the oil again this time
10-40 semi. Still got the tapping. Can't see it being the chain, think it must be
oil related. Valve? Any ideas? The car has done 125000 miles, only got
service history up to 80000. Cheers

Answer
Brett:
  It is important to distinguish between "tapping" & "knocking". I define the former as a relatively "light" noise impulse that is unrelated to engine loading whereas "knocking"  is a 'clunkier' sound whose nature changes with engine loading.
 Your description appears to be more like "tapping", in which case the problem lies in the valve train as a loose/worn/stuck tappett.The suddenness of the problem's appearance suggests a stuck tappet due to some breakaway contamination such as a dirt particle.
 If however, you feel that "knocking" is a better descriptor of your problem, then that is a more serious problem related to the lower bearings, but this is most unlikely.
 I would suggest removing the valve cover and inspecting the valve train as a start.
 Chain related noises typically are more like "rattling" and change characteristics during acceleration and deceleration cycles.

   Hope this helps,
                  Jerry