I believe this is normal. It is the variable valve timing (VVT) kicking in, so sort of like V-TEC. On a stock car it is probably fairly quiet but if you get a tune it will make it far more aggressive and it sounds like a tin can shooting around in your engine. Kicks in around 3500rpm-4500rpm where the dip is.
I complained in the past about this when I got my OFT but people seem to be OK with the tradeoff.
Some info about VVT:
http://www.ft86club.com/forums/showthread.php?t=47263