I have yet another crappy expansion cap that let's out a slight hiss..... you wiggle it loose and tight a tiny bit and the hissing stops... these things are really crappy!
So I remembered on my old F 'Meg' I had a Freelander cap instead - it was yellow and all plastic, but I don't remember it ever hissing! There's an eBay link below. I presume there are no drawbacks to using one of these? (other than the extra few mm height causing the flap to not be so flat)