speedo making noises

not sure if this is the right forum, but...

my speedo recently started doing this crazy bouncing back and forth and making a buzzing/grinding/whirring noise. it's intermittent. it usually starts if i go over a harsh bump at freeway speeds, and stops if i pull to a stop or just beat on the dash long enough.

the sound is definitely coming from in the dash, so could the cable have come loose from the gauge? i've never dealt with this kind of thing before, any advice would be awesome!