A320 flight crew reports slight vibration in the cockpit floor and electrical fumes during descent. An emergency is declared and normal landing ensues. During taxi-in the vibration increases then stops at the sound of a circuit breaker tripping.

Narrative

At a cruise altitude of FL380 I noticed a slight humming sound coming from behind the First Officer seat. I called the A Flight Attendant located in the forward galley to ask if she felt anything; her comments were; 'I hear the sound and there is a vibration on the floor when I stand in front of the forward galley'. I notified Dispatch via ACARS of the humming noise;and sent Maintenance a log item via ACARS. On descent the humming noise increased and an electrical smell was noticed for the first time in the cockpit. Called the Flight Attendant again to ask if she smelled anything unusual; her reply was: 'I don't smell anything back here'. There was never any smoke present associated with the noise or electrical smell. After switching to Approach Control I declared an emergency with ATC; requested priority handling and fire rescue equipment standing; we got cleared for the visual approach. In the mean time; I called the A Flight Attendant again to inform her of the emergency and directed her to conduct a Cabin Advisory. Upon landing; we requested an outside visual inspection of the airplane for possible smoke or heat signature; none was noted by the fire personnel. We taxied to the gate followed closely by firefighting equipment. Enroute to the gate; the noise increased as did the electrical smell until a CB tripped and the noise stopped. Upon arrival at the gate; we where met by firefighting personnel who found no heat signatures in the cockpit. After that the passengers deplaned normally.