I was talking to someone about the rattle/knock my bike is making and they suggested that I might just need a valve adjustment.

I have 15K miles. Is this so?
How much should it cost to have it done?

 

It's possible, but not one bike I've owned needed the valves adjusted.......after the first service. The first one is critical, but you are well past 600 miles so........I would bet more that it's the cam chain tensioner. Does it get better when it warms up? If so that is likely your problem. It MAY be the valves........but not likely. I couldn't tell you what it costs to have it done as I've never had to pay for it to be done........get the service manual and check them yourself. Getting TO the valves is the hardest part. After that.......you're clear sailing, but like I said, I would bet on the CCT first.

 

+1 on the cct as well.

 

I am quite familiar with cam/timing chain noise as my f1 had that issue and was told it was common.
This sounds a bit different than that, but honestly can't say. It seems to come from the left side of the engine right on the outside as best I can tell. Isn't that where the chains are?

Is it something I should worry about you think, or something to just get used to like my old one?
I mean given this is a 12 year old bike that has been treated god knows how. It runs good and doesn't give any other trouble. Just don't want anything to lock up while going down the road...