I have recently added a 2007 Fender USA P Bass to my collection. I am getting a really strange sound when I fret the 7th fret on the d string and to a lesser extent on the 5th fret. It sounds like I turned on a Octave effect. I am running through a Tech 21 and into a Hartke amp. I don't hear it at lower volumes but at concert volume it is very noticable. Has anyone ever had this happen? Any ideas what could be causing it? The bass is kind of a frankenstein. 2007 USA Highway One P Bass neck and a 2007 Fender standard P Bass string thru body. Thanks for the help.