Strange Noise at about 1500 rpm

This past week or two I've have began to have a strange noise, rattle, resonance, type sound coming from what sounds like underneath rear area of the truck. And then other times I begin to think it is coming from the engine area, or the engine itself.

The sound is not constant. I can only make it create this vibration/rattle/ping when under slight load/acceleration when the RPMs are at or just about 1500 rpm.

The sound can be created if I am barely on the gas around 1500 rpm, no matter the speed or gear. If I accelerate past 1500 semi fast, no noise, I have to just be easing through 1500 rpm or barely giving gas around 1500 rpm. It appears to be getting louder or worse, or I am more aware now than ever of it.

I have tried to record the sound. Most of my research on the forums has pointed to the airbox going through the fender well or u joints. But I have checked and this does not seem to be the culprit so far...

Take a listen...I've tried accelerating slowly from a stop and you will hear the noise distinctly (3 separate recordings in one).

Any thoughts, ideas, or someone who has heard this noise and knows what it is?

Noise in question is around the 6-7 second and 18-20 second marks - have to listen closely.

http://youtu.be/kFflOipmDIM

 

Changed video recording to one without big rig beside me at light.

Noise in question is around the 7 second and 20 second marks..

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Well, after a week of messing with what I thought was every possible loose bolt, shield, and the air intake where it meets the inner fender, and after wasting my time to post up and record the noise, I think I have it solved..

The damn spring bolts that connect the mid pipe to the cat back had backed out enough the the connection/springs were rattling/vibrating at a specific harmonic to be heard fairly well in the cab. Hope that cures it, seemed too on a short test drive.