intermittent engine knock sound on 1st gen

So in the past week I have noticed an intermittent knocking sound coming from the engine, I only notice it when I am stopped in traffic. Sometimes it is louder than others.

Truck is a 2004
V6 3.4
114,xxx miles
fresh oil change a few days after this started
only engine mods are vortex throttle body spacer and K&N CAI
truck has a lot of lighting but only headlights are on when this happens
lifted
armored etc

I was able to get it to do it by luck today while I videoed. Any help anyone can provide would be very much appreciated, it is my daily driver and I am starting to stress on this. Thanks!

http://youtu.be/BSZcPiA8hyc

 

Experimenting today, if the fan is off I can't get the sound to appear. Fan on and it makes sound intermittently. Always right when I turn the dash knob on and stops as soon as I turn it off. So the fan is either putting a load on something that is making the noise or something in the fan/AC/heat system.

 

much better audio on this video, about 11 seconds in I turned on the HVAC cabin fan to low and the knock starts. https://www.youtube.com/watch?v=slPLi-nzXsU

 

and the answer is........... Failing AC compressor! took it to a toyota tech who is a friend for a free diagnosis today.

 

I'm pretty sure mine was doing that as well. When I first heard it, I was thinking a bad rod. FWIW, mine went another 100k before I took it off the road beacause of the frame rusting out. Still ran good and cooled good. I wouldn't fix it until it finally goes entirely.