Valves Rattling ????????

I have a 2005 Toyota Tacoma 2WD Double Cab, just purchased with a 7 year/100,000 mile warranty. Recently Noticed when I accelerate moderately, it tends to have a rattling sound coming from under the hood, it's significantly loud. But tolerable. Was wondering if it was the valves rattling or what? If I let this go on will it cause problems in the future, or is it normal in toyota trucks or what? Let me know, I'd appreciate it. Tim