[Landseer]

That is the nature of the fuel pump whine, can be intermittent, probably as it passes some sort of foreign matter. Best practice at this point in car's life would be to remove the pump and filter, also then the tank, clean the tank, replace the screw in strainer at the base of tank, the fuel supply hose leading to the pump, the pump and the after-pump fuel filter.

Complications can enter the mix if the embedded fitting, into which the strainer is threaded, happens to break loose from the tank. That has never happened to me, but to others.

In resurrecting one of these cars, I have always replaced the strainer, pump and filter canister.

BTW, fuel line fires in engine compartment are a known issue with 928 and 944/51 due to hose deterioration that may not be evident. Suggest you do some significant reading/inquiry on this if you've not already addressed it.