I don't know about a roadbike but my trackbikes don't have expansion bottles but only overflow bottles, so there is always some fluid in the bottles but I've never heard any hissing (have you checked the bike for snakes ? Sorry, but am watching "Anaconda" right now)
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Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom