Piston Knock or Throw out bearing?

In the long process of figuring out what kinda shape my newly purchased 99 GT is in. I have noticed that while the engine is cold in first gear at about 2200 RPM's i hear what sounds like a piston knock like do do do do do do. Then after the engine gets up to temp the sound goes away. So far ive driven it several times and it always does the same. Car doesnt seem to be having any issues with the other gears just First. Which makes me wonder if it could be the TOB on the clutch. From what i understand piston knock is fairly common on these cars and not something i should worry too much about. If it was something more serious like a bad rod or valve knock I wouldnt guess it would go away upon the car warming up to middle temps correct?

Ya mine doesnt do it on Start up or at idle. Just when i reach about 2k in first. I usually just shift b4 that until the engine is warm.