Many people have pointed the buzzing/ humming sound from the speakers are caused by a bad contact in the subwoofer.
Inside the subwoofer is a large connector where all the audio is routed. Overtime, the metal pins in this connector oxidise causing bad contact leading to the irritating hum.
Purchasing a new subwoofer solves the problem temporarily for in time, it will develope the same problem. After so many years, this part is no longer available from Apple and hard to come by.
Ever so often, people put up their TAM affected by the dreaded hum giving you a chance to pick up one inexpensively.
Don’t dispair if your beloved TAM is exhibiting this problem for it is not that difficult to resolve this problem by following the steps in our guide to take the subwoofer/ powersupply apart.