Licking the bars of his cage?

Hiya, so recently I've noticed my chinny licking the bars on his cage, he goes up and down them almost obsessively for 5 or 10 mins each evening, but I can't tell why he's doing it? He's never done it before these last few days. I've tried putting more toys in his cage incase he's bored, but he's still licking the bars... :hmm:

If anybody's experienced this before, do you know why they do it? Is it something to worry about?