98 2.2L Ticking/Chattering

I've been hearing a ticking or chattering noise coming from the top of the engine for quite a while now, notably at low to mid RPMs. The car is a daily driver and pretty much being driven until it dies (120K on it now), but I was wondering if anyone else had this same problem or knew what it could be.

I suspect it's a valve problem but want to poke around before I throw any money at it. Like I said, it's been going for a while and I haven't noticed any difference in performance.