Hissing Peugeot 307 1.6 (MY2003)

[TomTom0507]

Hello,

I have been using my dad's car recently (a 2003 Peugeot 307SW 1.6 petrol), and when I start it up, there is a loud short hiss. Im not too sure where it is coming from, but it only happens when I start it up after a few hours of non-use. The ignition also takes a few seconds to fire up the engine.
Anyone else experienced similar issues or ideas on what it could be? No leaks found.
Thanks in advance

Tom

[TomTom0507]

Thanks very much for the reply. This is very helpful! I will look more into those areas. :-)
Many thanks