Watch speaker hissing/white noise receiving notifications
Thought I'd post this just in case anybody else ever has the same issue.
All of a sudden this morning, whenever it received a notification, the speaker on my Watch was making a hissing/white noise type of sound from the speaker, as well as the normal haptic tap. My Watch is set to use haptic only for notifications - no sounds at all, so I'd convinced myself it was a hardware issue and I'd need to trudge into town on Saturday to the local Apple Store, but did a web chat with someone from Apple anyway.
Thanks to the lovely Heather, the issue is now resolved and all it took was unpairing the Watch, a reset on the iPhone (hold down wake/sleep and home buttons until the Apple logo appears) and then pairing again.
I had to add my debit card to Apple Pay again, but other than that I didn't lose a thing as a result of unpairing ...