Help! Weird engine noise

Hi,

I have a 2002 Nissan Frontier with a four cylinder engine. It has around 56, 000 miles on it. I started hearing a weird noise coming from the engine that sounds sort of like detonation from cheap gas, but not exactly. It is more like a clatter/rattle. It does not make this noise on idle, and only does it below 40 mph, especially when accellerating slightly. It is at it's worst when the transmission first shifts into overdrive (about 35 mph). If I accelerate moderately, the noise disappears. I do not hear the noise if I floor the accelerator. When I get above 40 mph the noise also disappears. Could this be the timing chain? Any other suggestions?

Thanks,

John