Front teeth nibbling

Question
I have a 1 1/2 year old lab which I have never had a problem with chewing (furniture etc.). Recently she has begun this fast/tiny nibbling just with her front teeth on the furniture, bedspread etc.  It doesn't leave a mark but leaves it wet with saliva.  Any ideas why she has started doing this?

Answer
Does she need her own appropriate toys to chew on?  
Is she bored?  
Does she need more exercise?

When she starts chewing on anything you don't want her to chew on, you should re-direct her to something appropriate. (Instead of saying, "No, don't do that," say "Do THIS instead.")

Give her toys that are challenging, that take some thinking:
https://www.premier.com/store/Products.aspx?cid=3
http://www.marinhumanesociety.org/Behavior/Handouts/KongsHomeAlone.pdf

A tired dog is a good dog.  Be sure she's getting enough exercise.