Sorry but none of these solutions work. I spoke to Samsung. It's a glitch from a recent software update. You have to take phone in to the store and get it reflashed and diagnostics. It's a huge problem with s9 phones happening right now. Call Samsung for a repair ticket
I think I finally figured this one out. Just tried random settings, and turned of "NFC and payment" (I'm a bit fuzzy on translation here). Apparently it's my credit card and cellphone communicating or something. Turning of the setting stopped the sound. Whenever I usually use my phone my credit card would lie straight behind my phone, but when I took it away while the setting was turned on it also stopped. Either way it worked out for me, hope this helps.