A disposer hums for two reasons:
1. The usual reason is that it has an object jamming the blade. Unjamming requires a large screwdriver or crowbar stuck into the disposer and twisted to rotate the grinding plate breaking loose whatever is lodged. The manufacturer supplies a useless unjamming wrench that seldom works.
Amateur Method
Professional Method
Trick of the Trade: Be careful not to twist so hard as to loosen the disposer from sink flange. A jammed piece of metal takes considerable force to release.
2. Occasionally the grinding plate may turn freely but the disposer still just hums. This means the disposer motor is shot and a new unit is needed.
Trick of the Trade: Don't even consider disassembling a disposer. At best you may succeed in replacing a tail pipe or trap.
(On a reversing, expensive disposer it may be possible to replace a bad motor reversing switch. This would only apply to $300-400.00, ISE 77 unit.)