This is a side Easter Egg during the campaign for Black Ops.

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Thundergun in Campaign
Much like the Ray Gun in the previous game, the Thundergun can be unlocked in Black Ops by performing the following specific actions on the level "Numbers." After the interrogation ends, look for six canisters.

Toss grenades at the canisters to the rear left and rear right. Shoot the other canisters until smoke is released.

A black tape will appear out of a cassette deck. Grab it with the action button. Leave the room and head upstairs. Kill everyone and move to the rooftop and jump to the next floor.

Weaver and Clark will move the fridge revealing a weapons cache. Here you'll find the a cassette player. Hit the action button here and you'll put the tape in the deck.
You should hear music and the screen will shake and shortly the Thundergun will come out of the wall.