What would cause 1991 Volvo fuel pump to make a buzzing sound that cuts on & off like Morse Code?

The fuse box refers to it as the fuel pump that feeds the fuel injectors,the Haynes Manual refers to it as the main fuel pump. Is this outside pump being overworked due to a failed in-tank pump causing outside pump to make buzzing sound?Haynes Repair Manual makes some referance to this posability along with 1000 other posibilities.No Volvo mechanics in my area. Anyone know what can cause this buzzing sound or where I should start at diagnosing the problem?What would cause 1991 Volvo fuel pump to make a buzzing sound that cuts on %26amp; off like Morse Code?
in tank pump primes the main fuel pump, if in tank pump fails main pump is overworking and noisier than normal this is most common fault

note all Volvo's are prone to noisy fuel pumps when they get older. what model is itWhat would cause 1991 Volvo fuel pump to make a buzzing sound that cuts on %26amp; off like Morse Code?
your pump is telling you its on its way out.

240, 740, 760, 780...etc same answer.