[Boxercox]

Hello,
Holly is my fabulous, brilliant, wonderful four year old brindle bitch Boxer.
In the last few weeks when she is settled for the night she is grinding her teeth. She seems to grind them again and again.
I have very carefully checked her mouth, teeth and gums and can't see anything wrong, or anything stuck in her mouth. She does not seem to be in any discomfort and as soon as I say something to her she stops.
I would welcome any suggestions as to what this might be, could it be something she is doing as matter of habit?
Any help would be most welcome.
Thanks
Boxercox