Been having this for a few months now and cant think what it could be

When you start it, when cold it will rev up to 1100ish rmp and sit there till it warms up abit, which is normal... But if you rev or drive the car, the rev will sit up at around 1500-1800ish rmp for a good few mins but its fine after its warmed up.
It only does if when starting from cold and the car hasn't been drove that day... And it also make the same kind of noise as a broken secondary air pump. but had a look and my car doesn't have one

Anyone got any ideas what it could be?

Oh and the engine is an AGU, 100% standard.