Flea does this occasionally for no apparent reason: almost as if she is "smacking" her lips or chewing something invisible. Is this a tic or something more serious?

i can't really help but I didn't want to read and run

have you got a video? some people might be able to help with a video

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We have a birman girl (6) who does this and has since she was little, asked the vet about it and recently, he said nothing to worry about, is just one of those things she does. Is a horrible noise though like teeth grinding.

For peace of mind, might be worth getting yours checked out to make sure there's no issue with teeth, mouth especially if she has only recently started to do this. Also can't remember of the top of my head, there is a condition cats get where they can have 'smacking'

Firstly, how old is Flea? While the occasional episode is probably nothing to worry about lip-smacking can also be due to stomach acid, or dehydration. With an older cat it's definitey worth keeping an eye on as it could point to renal problems.

Thanks.
Flea is just under two years old. She had a bad start as a pregnant feral at 6 months and cared for her own litter and another cat's litter. I had her spayed when I adopted her a year ago. I assume her diet on the street was not good. However, the vet has run tests on her and she seems to be in good health now (aside from some alopecia).
The smacking doesn't happen very often, as far as I can tell, about once a week or so. She hangs around me almost constantly when I am home, so I would have noticed if it was a continual thing.
She is due for her annual checkup soon, so I will put it on the list to ask the vet.
I hope it is just a tic, like Nunu's grunting when he grooms

In that case then it's probably nothing to worry about! Don't forget to ask vet to reassess the situation re the alopecia-doesn't sound hormonal in origin to me!

Cats' smack their lips when they are feeling nauseous, pretty much the No.1 cause vets see. Could she have just eaten something or had a medication shortly before the smacking starts?

And acid indigestion a close No.2.