Well i started my car for the first time after 2 or 3 weeks of not moving (after charging the battery) and let it idle for 5.

Just before shutting down, i gave it a slight increase in revs (1800 max) .

after doing this i noticed above about 1100 a hissing noise, as if someone was letting down my tyre. The noise was coming from the engine bay.

I had no time to get someone to inspect it, so i turned it off and left it for today.

any ideas what it is?

Maybe Vacuum leak or air filter not in or not seated properly. Get someone to listen while you rev or vice versa.
Paul

Possible manifold gasket leak.....Try tightening the inlet manifold and see what happens.

well, after a closer inspection, it appears to be coming from PS pump, but still not 100% sure. I have had some issues with the pump before, so i think i may aswell put a new one in..

So you left it for 2 or three weeks.... hahah you have a snake under your accelerator pedal!!!

lol jks, try for exhaust leaks? easy way to notice is when you turn your car off you will hear it, or if you get someone to hold a rag over the exhaust tip you can find the location.