Hmmm...
An electric pump shouldn't change it's sound. They're either on or off, the pressure regulator may make a buzzing if it's going bad...
It may have an integral press reg/return...

Now I'm curious, I'll look at the manual and see what I can see!
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OK, now I'm back.

Our fuel pump and separate press reg are mounted transversly on the crossmember (bottom area of tank bulkhead?).
So, I'm thinking you are most likely hearing the regulator, but I have been wrong before, it very well could be the pump.
Good call!

Since they are both mounted together, you should be able to add a "softer" mount. Something more flexible, to help isolate vibration/noise.

The serv man says that to access the pump & reg all you need to do is disconnect the battery (safety first) and remove the splash guard. (under-body panel)
If you are going to open the fuel system you MUST relieve the system pressure first! 54-61psi fuel spray = blindness, big fireball, ruined day etc...

I don't know how you can trouble shoot this if it only does it when at speed. Maybe you can look at the mount and see if a soft mount is even feasible. It may be as simple as placing a piece of rubber gasket material between the crossmember and the mounting hardware, or between the pump and/or regulator and the hardware, or between the... you get the idea.

I have been a bit curious as to what it looks like under those panels, I just might take a look myself this weekend.

Matt