I think we all interpreted your words incorrectly. From reading your last post i'm leaning towards a respiratory infection. It sounds like what we interpreted to be a 'hiss', or air being forced out when he jerks his legs and head in quickly, is really wheezing from congestion. Reason I think this is because they don't sit there and hiss for no reason. Some pet stores have some sort of policy where you can bring a sick tortoise back and have their vet treat it. It might be a good idea to do that.