Some fuel pumps hum more than others. When is the last time you changed the fuel filter, may be dirty. It may be pushing slghtly harder, you say sits for a long time may have slight build up. But the hum is normal.
__________________ 89 LX vert. Auto to 5 sp.(t-5WC)Hurst short throw 3;73 rear,trickflow cover. Trickflow HCI street heat stage 1 ,24lb.inject. 70mmbody,75mm prom mass air. shorty headers highflow cats, flowmaster. Adj.fuel reg.190fuel pump. underdrive pullies,cold air fender. blue hose and 3 row radiator, 180 therm. edelbrock high flow,SS fan clutch,blast coil and 6al multispark, blue wires, subframe conectors and strut brace. trunk-no rack, 93 sp.ed.spoiler, cobra R 17, rear disk brakes, G3130amp/alt | 
1991 Mustang Fuel Pump Buzz? 
