Well, I just installed one a few weeks ago that I ordered from Megan Racing and looks alot like the AF1 and setup identical to the Airraid FIPK.

I now during start up have this loud horn sound coming from under the hood. The RPM's stay up while this is happening (2k or so)...I gave it some gas and it goes away. Anyone with experience or help? It sounds like someone is through a pipe with a tight rubber flap on the end

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I have the K&N FIPK and had the same kinda high pitched sound. The instructions say'd what to do if this happens, so I kinda did that. On the KN, right before the inlet tube connects with the factory intake thing, thier is a 5-7" rubber hose kinda in a L shape, (comes from K&N) and cut it about an inch shorter to make the bend smoother. That is all I did and it worked. You can also reinstall the factory silencer elbow thing, and that should work. If I find some pics I will get them and might be easier to understand,

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My FIPK will be here tomorrow, will install tomorrow night. K&N doesn't have a kit for the 2003 SD, but I'm sure it will fit. If not the shop will take it back and provide a full refund.