Unfortunately, a fuel pressure gauge costs a lot for how much you'll use it, but its invaluable to diagnosing fuel pump problems. I've seen them for as little as $18. Make sure your vehicle has a spot to check fuel pump pressure, some don't have one, most do.
But, if the fuel pump makes spec pressure consistantly, then its unlikely its going to die soon, there may be some other reason for the noise. It may be something as simple as a rubber grommet or isolator wore or deteriorated and is letting noise transmit better thru the body.
And often fuel pumps just die immediately, with no warning, or work just fine making extra noise for a while before dieing and they can just as likely start making extra noise but still work fine for years. i.e. sometimes you can't avoid the crap shoot, they can give you no warning of failure, they can give what seems like a warnings and never fail.
Most fuel pumps are constant volume pumps, they use a return line and pressure regulators to change the pressure/volume available to the injectors. That is why the noise will remain constant regardless of rpm or engine load, the pump load is always the same. If the injectors need more fuel, less is returned thru the return line and vice-a-versa.
Some systems, and I think some XJ's are like this, do vary the fuel pressure a bit, they usually have a vacuum line connected to the pressure regulator, so fuel pressure may go up and down a bit with engine load, that may or may NOT effect the pump and its noise its make.
