Hello please

Last few days I've noticed my car shuddering when accelerating. After looking into the engine, I find one spark plug loose. I refit and take car a run. Engine mgmt light comes on. So I connect it to a obt2 and check error. Says misfire on that same spark plug. So I buy new plugs and refit. STILL HAPPENING!!

at an end now. Don't know what else to try

 

Also getting an air hissing when sitting idling then rev the car. Like there is air getting in somewhere...?? (Almost like a dump valve noise)

Have you tried clearing/resetting the DTC from the memory and then going for a drive?

I've a 2003 1.6 Focus and mine hisses a little when you turn the car off. You hear the engine settle and the a 'hiss' similar to a dump valve sound.
Had everything checked for leaks but couldn't find anything.

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Bazz, mine makes this noise but I think it is just the noise of the sudden inrush of air through the intake.

If it is only misfiring on one cylinder then I would check the plug gap, ht lead but also fuel supply to that cylinder.

Aiui from my own research into my general misfire the coil pack uses a waste spark system, if it was faulty it's likely to misfire on more than one cylinder (there are two coils, so two cylinders will spark at once, hence the "waste spark" notation. There is always the possibility of an internal damaged section leading to one electrode on the coil pack bit I would say unlikely if the outside of the coil pack is intact and contacts clean (coil packs are £30ish new, I have an aftermarket Hella coil pack on mine now).