Valves rattling?

I have a 2008 4cyl with about 55k miles. Ive had the vehicle about a year now but I just recently noticed(because I had the radio off) that it sounds like I am getting a valve rattling sound. This sound I hear only occurs when I am going between 40-50 miles mph and with the rpms around 1200? It seems to be more prevalent when there seems to be a load on the engine. For example crusing at about 45mph,lower rpms,then start on an gradual incline on the road. this scenario is when the noise(very faint) is heard. Is this coming from my valves or other engine parts?

 

I would imagine its something else rattling, I would have some one hold the engine at the Rpm you hear it and go poking around for where the sound is coming from.
If it were valves making noise that would have to be out of adjustment of not getting oil could be a few more scenarios, but this a Yota. I bet its just something loose.

 

Try a higher octane gasoline. The key indicators have been bolded in your post above. I would guess it is an engine pinging from the extra load and low octane gas. Pinging will sound like something "rattling" when it is mild.

Run some Techron Fuel additive through the a full tank of gasoline 93 octane or better. I believe it will go away.