cat vomiting clear liquid

My 19 year old cat has been vomiting clear liquid in the mornings, especially if he hasn't been fed. When he vomits, he sounds like he's about to vomit a hair ball, but just clear, sometimes foamy, liquid comes out. He was throwing up a lot of hair balls recently, but that stopped when I started giving him the Sentry gel for hairball relief, now he just vomits the clear liquid. Otherwise, he seems to be doing okay. He's pretty active, especially when he's hungry, and he wrestles with my other two cats every day. I'm worried that he's sick, but since he's still eating well and is active I also think I might be over reacting. 

It might be nothing to worry about because sometimes cats do throw up liquid when their stomachs are empty.  However since your cat is 19 and he just recently started doing this, you should probably take him to the vet for a check-up.  Nineteen is really getting up there in years so he should be getting check-ups every six months, anyway.   That way you can stay on top of any trouble that might be brewing.  

I just noticed this is your first post.  Welcome!    And please post some photos of your old guy - we'd love to see them.  There is something really special about the seniors.  What is his name, anyway?