My 11 year old dog is randomly yelping in pain?

A few weeks ago my bet hound started yelping out in pain when anyone would walk past him, not even touching him. Now at random times (once a day) he has a 30 second fit where he's howling in pain. It's awful! We've checked him over carefully and can't pinpoint a specific location of pain. I took him to the vet and without doing any kind of test, he prescribed Rimadyl, thinking it may be arthritis pain. I am trying to be realistic about the dog's age and don't want to spend a lot of money on testing/surgery/etc. He has been my baby since he was 6 weeks old and this really saddens me. Has anyone else had this kind of problem, and what did you do about it or what was the diagnosis? Thank you.