what could be causing this noise?

see other post: You have "pre-injition" Basicly a carbon build up. You need to start using the next grade in fuel. You might want to go in for a "injector service". As cars age this happens. After the injector service I would recommend that you change your plugs. And this will cure the "ticking". If that is not the problem, You could have a exhaust manifold leak, that is common. You might need new bolts. Or if that dont work, You could also have a bad spark plug wire. All these will cause "ticking" upon acceleration. I would recommend that you get it looked at, the mechanic will be able to direct you on which issue it is. Best of luck.