pressure checking coolant is an easy way to find a problem but you could put more sh*t in the cylinders. leak down test is better - check out a how to vid on youtube
this is a good one -
http://www.youtube.com/watch?v=QNf0IOIv ... re=related
http://www.youtube.com/watch?v=GPnm3YwP ... re=related
if you have no bubbling in the radiator but a cylinders not holding pressure, the smoke could be oil being burnt in your cylinder. which could go along with the knock noise you're hearing. but a blown head gasket or cracked head can also develop a knock. test it out and spend some money on the leak down tester its a good investment if you plan on working on cars the rest of your life.
harbor freight has leak down testers for 50$ can get them cheaper on amazon/ebay. all you need is a compressor that will hold 100 psi, ask a friend/neighbor if you dont have one