s40 hissing and not starting

hi my volvo s40 2 litre petrol injection 1999 which has 120000 on the clock just wont start nice. it will be fine for a week or two weeks, then it will just not start when you try to turn it on.... it will hiss though, the starter motor will clunk a bit - at least something clunks - you can see the fan belt move at that point, then it just gives up and hisses!! After a bunch of attempts it gets going unhappily but once fired up it stays going and is as good as ever. no idea what is going on here at all, and nor does my mechanic.... please help, thank you

hi i have been on holiday for a while and haven't had a chance to check it out thank you so much for your response. i have checked all the standard things that you mentioned, unfortunately to no positive outcome. i am unsure as to whether the air con unit in the car should be causing this, mainly as the problem is intermittent, seemingly every few weeks it starts, and then just goes a way for a while. i have a breakdown service coming to check it out - perhaps there is a lack of pressure in the fuel pump - this was suggested to me recently by a friend. will let you know thank you

Could the hissing be coming from a badly fitting spark plug or one of the 2 manifolds?

Chris, i take you are thinking the compression is low on one pot due tot he plug being loose???? hence the crappy starting....... Guess a compression test wuldn't go a miss, certainly eliminate alot of possibles.... Best of luck finding it...

solenoid in the starter motor was jammed. got a new one, brand new s40 again!!

Glad you got it sorted and it was something fairly simple/relatively inexpensive.