Persistent Fuel Pump Noise?

[WorkingFromHome]

When I bought this boat, 2 years ago, the starboard fuel cell was bad, so I replaced the fuel pumps on both engines for the peace of mind. I don't know if the issue was there before because I couldn't get on plane, but I have always had a persistent high pitched electrical/whining noise above 2800 rpms, it sounds like a bad fuel pump. The noise isn't tied to the rpms like a belt, it comes on gradually and then doesn't change volume, frequency or pitch once it starts, all the way to WOT. It's not immediate either, sometimes it'll take 5 - 10 seconds before starting. It's running fine, but my family are hesitant to go far because it's very annoying and not exactly relaxing.

• I replaced the fuel/water separators and I cleaned the fuel pressure regulator screens.
• I tried running both engines off one fuel tank, then the other and then isolated and then off both.
• I've tested both pumps which seem to be working fine and I have even replaced both high pressure pumps (guess I have spares now) with new ones.

None of that made a difference, so it seems my issue is somewhere else in the fuel system, but I am at a bit of a loss of where to look next.

Any suggestions on where to go from here?