Hissing Noise After Engine Switched Off

[ub7rm]

Alright chaps, last night I got out of the car and went to change the oil filler cap for a new one I had bought, (nothing fancy, old one was broken).  So I switched the engine off, got out and opened the bonnet.  I noticed there was a hissing noise  coming from the right hand side of the engine, sort of HPFP area.  It was dark and peeing it down with rain so I didn't investigate much but I was wondering if this was normal.  Something bleeding off pressure perhaps?  It was the noise of air / gas escaping from something.

Car runs absolutely fine, no lights or anything.  There was no smell of fuel or anything else untoward.  I wouldn't have heard the noise unless I was in about the engine - not that loud.

So is it normal or does anyone have any ideas?

[Poverty]

Take your car out for a thrash, like really thrash it, get the downpipe glowing, and then let it idle outside your house. Keep an eye on the temp gauge throughout this period and see if it remains stable

[Poverty]

could also be pcv related

[SKAVIA]

Could also have been the rain coming off the hot bits off the engine?

[ub7rm]

I will take it for a thrash for medicinal purposes at the earliest oppotunity 

Though I have to say the temp doesn't seem to be an issue in everyday driving, coolant hasn't budged a mm in the 2 years I've had it now either.

Yeah could be PCV related - no issues with drivability, idle or anything normally attributed to PCV so not sure, it might be starting to go bad?

Good thinking with the rain - certainly can't rule that out, but it was a very steady tone, not like drips hitting a hot surface.  Sounded a lot like either air leaking out at pressure or air leaking into a vacuum ...

Opened the bonet last night after stopping the engine and couldnt' hear the noise.  Could be nothing but will be keeping an ear out in the near future.  Thanks for the pointers 

[Poverty]

Before you changed the oil filler cap, where you driving spiritedly?

[ub7rm]

No just the normal commute, last 10 mins or so is though traffic to my flat ...

On a tenuous link, the oil filler cap is quite stiff to get off, there is a vacuum holding it in place.  I assume this is normal?  After the first time taking it off its pretty easy (until I start / stop the engine again ...) So I assume its vacuum pressure making it difficult to turn.