I have a very slight knock only at very low RPM's on my 90 YJ 2.5L. I have tightened the exhaust manifold down and that helped a bit but, it isn't gone completely. I had assumed it was an exhaust rattling but when I climb underneath and shake things around I am unable to replicate the sound. I read on another thread about a timing chain knock. Could this be where my problem is? Does anyone know how I can identify it?