I just changed my fuel pumps on a 1987 420 because of a leak rather than noise.
If you do a search you can find the pictures I took of the area.
The pumps should be at the passenger side rear between the wheel and the differential.
I would go under there with the car in idle and listen or touch to feel for vibration to see which one is making the noise.
Fuel pumps at FastLane are about $202 and each comes with a new check valve.
Somewhere I read to also change the fuel filter when replacing a pump.