Did it start sounding like a diesel after you adjusted the valves or did it develop after a while? Perhaps you did not tighten down the locknuts enough and they have since slipped.
The way I adjust my valves I use the feeler gauge, and adjust the valve to where the feeler gauge has just a little bit of drag on it when i'm pulling it out. The way I do it, the next size up doesn't even fit. I don't know if that is a bad way to do it, but it has worked for me for about 3 years now, and my engine definitely does not sound like a diesel. (although there is some light ticking)