That's a pretty big jump in shim thickness, but depending on how long it had been since an adjustment, I wouldn't say it was out of the question for a still properly functioning bike. Luckily it was on the loose side and not tight. That's when the really bad things happen.
If you only hear the noise after it's warmed up, it's not likely to be shim clearance issues. These you normally hear when the engine is cold, then they lessen in intensity as the metal expands when the engine warms, and the clearances tighten up a bit. My guess is you're not hearing valve noise.
Also, if it's after 6000 RPM, many of these engines have a characteristic "rattle" in the 6-8k RPM range. Just do a search for "engine rattle" and you'll get bombarded with results. There's nothing to worry about if that's what you're hearing; sounds kind of like BB's in a can rattling around.