2005 STI (5,000 miles)
COBB AP stage II
COBB turboback exhaust

Ok, I had these mods on for about a month now. It was just last weekend when I noticed a weird knocking sound whenever my rpm drops below 3000rpm, then it would stop when it was at idle. The knocking sound is very consistant and loud (maybe 3 knocks per second?). This happens whenever my car is moving (Acceleration) and also when I'm at a standstill, neutral, reving to 3k rpm. So I stopped, opened my hood to see where the sound was coming from. It seemed like it came from somewhere near the intercooler and the intake manifold. And the sound wasn't it one clear spot, the sound seemed to sway from both sides as if there was a rock in the intercooler going back and forth. so everytime I rev to 3krpm, once the needle falls, from 2700rpm to 1500 rpm, the knocking noise occurs. So I tried holding the rev at 2500rpm and the knocking was consistant and continuous, until I let off the throttle. After 3000rpm, the noise seems to stop or, I just can't hear it. The car runs perfectly fine and FAST, no boost problems.

Could it be some part of the timing belt? cushioning belt? rapid throttlebody closing and opening? engine mounts loose? pistons? sparkplugs?

Please help me out, really scared here.