The sound I get is air bouncing back after hitting the throttle body. I have the lowly D1 and at idle it is ridiculous how much air it moved. Even with the bypass I would read ~4psi (needle bouncing) on the intake side of the TB with the throttle closed (at idle). The only time I get the rythmic sound was when I had the engine up near redline and closed the throttle slowing back down. At that point the bypass can't keep up with the amount of air trying to flow through it. Moving the bypass past my intercooler and near the intake lessened the effect though since the air had a shorter bounce to make before escaping. Is that the best place to put it? I felt so since the air kept moving through most of the system keeping initeria up once the throttle opened again. The downside would be that hot air continues to move through the intercooler even while not on boost. With air to air there is so much excess cooling air moving through the intercooler I felt it wouldn't matter. I gotta look back at you pics to see where you ended up putting your bypass/blowoff.