Very likely the cam chain.
That was my first thought. If resetting or replacing the adjuster doesn't work, I'd recommend having a mechanic do the work. Unfortunately, getting at the cam chain is rather involved as it's rather deep in the engine.
While it is possible to get a manual cam chain adjuster, it's probably best not to make that a DIY as if you over or under do it there can be issues. And the instructions that I found when I wanted to do that were somewhat less than clear to me. On top of that, some of them can be tricky to get properly installed so that they don't back out.