M62 mis- fires

[peaktorque]

I.v been testing a few V8 E39's, there is one thing I have noticed on all the ones I have looked at. The four I have looked at all seem to hesitate around 1500 rpm when you 'blip' the throttle, it sounds a lot like a mis-fire, but this is consistant on all of them and they all rev freely after. My dad has a 4.4 X5 and this does it as well. Is this normal? What causes it? I should also mention that I have only noticed this on cold engines, not sure if issue persists once fully warm.