Here’s class D audio amplifier pumping remedy from MAXIM Integrated, power-supply pumping is a problem that occurs when playing low-frequency audio signals through a single-ended output. Link here (PDF)
This application note explains what power-supply pumping is and how it occurs in designs that employ a Class D audio amplifier with single-ended output loads. The article presents three design solutions that will reduce the problem. Mathematical equations show that use of power-supply capacitors greater than 1000µF greatly reduce the phenomenon.