Yep - I think that's what it was - a day or so before posting the question, I put some grease on the joint entering the wing, but it was awkward to get to and didn't seem to make any difference. Then a couple of days later, it seemed to ease up and now I don't have the problem at all. I found an easier way to lubricate it is to use some motorcycle chain wax in a spray can - that way I can get the lube in easier.
I'll keep an eye on them.
Matthew
