knocking or tapping on outgoing calls

I'm having the same issues. I switched from AT&T to T-Mobile a couple of weeks ago. I started with an IPhone 15 Pro Max. I started hearing a heartbeat sound after the first couple of days of using it. It  intensified into the sound of knuckles tapping on a wooden surface. I called Apple customer support and they couldn't find the issue. I just switched out the IPhone to a Samsung 24+ yesterday and the sound is back. It sounds like the devil himself is driving a nail in my head. It's driving me crazy. I noticed after reading your post that it only happens on outgoing calls. Sometimes it's subtle and other times it's very prominent! I'm going in this Tuesday or Wednesday and canceling my service with T-Mobile. Saving a few bucks a month is not worth losing my sanity...