[quote name='Junkyard Rocket' timestamp='1375945320' post='2167632']
I had something similar when I first bought my CIJ mustang when they could be picked up for £300 secondhand. As soon as I got it home I took off the roundwounds & put on a set of thomastik JF-324 flats, which are just as light & floppy as my favourite long scale 344s. I did as a good a setup as I could, but there was awful fret buzz around 7-9th frets which made the bass fairly unplayable. It seemed that this was due to the lighter strings being unable to pull any relief into the neck. I took it to the Gallery & the diagnosis was that the frets needed to be levelled. A fews days later I picked it up and it was perfect - no buzz or choked notes at all. The frets were leveled, crowned & the edges smoothed, and surprisingly there was now the ideal amount of relief in the neck.
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I had the frets levelled etc and bearing in mind the bass is almost new, it still rattled.
Maybe the strings were lower tension or possibly just knackered, who knows but it's lovely now