I have exactly the same problem (?) with exactly the same disposer. It's been doing that ever since it was installed (since June or so), so I'm just continuing to ignore it. For some reason it's just not starting immediately, as we would expect. Might be worth a call or e-mail to Insinkerator.
These disposers use a centrifugal switch to switch the starting capacitor in and out of the circuit -- I forget if it's in when starting or out, but if the switch fails, the unit will sit there and hum forever. It will hum briefly if the shaft is locked and then releases, but we both know the shaft is free. I tore apart an old Badger 5 that was humming a while ago, and you wouldn't believe the crud inside the motor that was keeping the switch from moving. I cleaned it up, put it back together, and it worked fine.
I got curious enough to send a request off to Insinkerator's "Disposer Doctor". I'll let you know what they say if/when they reply.