Hi gents,
Perhaps this should be in Blueprinting & Bondo so apologies to Andrew in advance if so. I need some help as I'm flummoxed
. Bought an Aria IGB68/5L, a rare bird, & after lowering the action, truss rod straightening, string change to taper-wounds so I could get the action lower as the bridge wouldn't let me go any lower, this bass has some nasty, b5 overtones 
happening on the D, A & some E string notes around the 14-24th frets. No particular pattern or consistency, as in buzz & the like, just a clearly 'out of tune' unintended double stops that sound very 'Twilight Zone meets Aliens in Roswell-ish', 
without an FX pedal. I don't recall if it was present before I started fiddling with it but I've been setting up my own basses for 30 years so not a newbie, though not a pro luthier either. Any ideas/theories/humor at my expense welcomed.
It starts on all strings bar the G & gets more obvious as I move up to the 24th. I've never experienced anything like this in 32 yrs of playing, & haven't heard of it either. I use very low action but play with a very light touch, ala Gary Willis, so I expect some buzz at times but this is a conundrum!