I pulled out my old Yamaha BB404 and decided to get it all cleaned up, which hadn't been played in years. I bought some of the cheapest strings on Amazon ($10 pack 4 strings) and set the bass up perfectly by the books to specs with the correct neck relief and action (Yamaha officially told me it is 3mm on the 24th fret). Everything played nice, except for a persistent buzz on the A string 5th to 7th-ish fret area. If you look everywhere on bass forums/Youtube about issues with fret buzz the answers you get are always based on "it needs to be set up correctly" and if that doesn't fix it "the frets need to be leveled/polished/etc". For weeks I just couldn't figure this out and was on the verge of getting the frets leveled when I noticed that A string had a noticeably visible amount of movement when plucked (large arc). Way more than on my other bass. Luckily I had my old strings so I decided to test that theory and I removed the A string only on the Yamaha and swapped it with the old A string. The fret buzz problem went away immediately. I speculate it has something to do with the gauge, probably lighter gauges are more prone to this. Only way to truly test would get light vs heavy and see what happens. If it isn't a gauge issue, then it is the fact they were $10 strings and you get what you pay for. I did post this to Reddit and another used mentioned that a bend in the string can cause this same effect. Regardless, it had to do with the strings. Either way, I'm writing this is a PSA for those of you who have fret buzz issues. If your neck relief/action are perfect and there isn't anything obviously wrong with your frets please CHECK YOUR STRINGS. Of course when you google search "fret buzz lighter string gauges" you will read replies that "don't be insane, string gauge does not cause fret buzz", reconsider. I googled endless on how to fix fret buzz and no one mentions the strings as a possible culprit. So much cheaper and easier to test your strings than blindly assuming it must be a fret issue or warped neck issue and going that painful and expensive route.