Engine Tap/clicking at 2.5 rpm

Hey all,
I have a 1988 saab 9000, manual trans, non turbo. I recently had my knock sensor replaced, after driving on a bad one for many months (this
was causing my engine to run very badly after around 10 minutes of driving). the day after getting my knock sensor replaced (as well as my idle belt pulley, and some bad belts), i began to hear a very click/tapping noise whenever the engine is at 2.5 rpm. The engine runs great, and is unaffected performance wize by the sound. While accelerating, the engine will click and tap once once hitting 2.5 rpm, but stop once past. when the gas pedal is released, it will click as it passes 2.5 rpm on the way down. during highway driving, if a speed is held at 2.5 rpm, the engine will click and tap constantly.
I have noticed that after prolonged high speed driving, the engine tap/click seems to all but disappear, but then returns the next day. This also may be my imagination, but the click seems to be less on rainy days.
i've been told that engine click can come insufficient oil on lifters, dirty fuel injectors, engine knock, and bad air hoses (i don't know much about that). most mechanics who have taken a listen to the noise say its not lifters. I used fuel injector cleaner on this past tank of gas, and while my engine seems to accelerate a bit smoother, the clicking noise still lives. the mechanic who replaced my knock sensor doesn;t think it is engine knock, but can't be positive.
If anyone has any ideas as to what this could possibly be, your help would be greatly appreciated.
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Turn up the radio and be happy a 18 yr old car still runs. Best rregards papa
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