Has this happened to you??? OAK

While measuring the intake on my Harman XXV to find a pipe to attached for my OAk system, I noticed a whistling noise when my tape measure was in front of the intake. Is the intake that sensitive that the width of a tape makes it whistle? I said no, must be a fluke. Well, I found a 3" pipe which fits over the intake, so you would assume there would be no air restriction, because the one Im adding is bigger in diameter. Well as soon as I put on a 6" in length (trial run) piece to see if it would fit it started to whistle again. What am I doing wrong? Help. Am I just listened to closely??

The whistle will not be present with a longer run I would think. Is the stove virgin? Under heated fire the draw is naturally lower too.