应该填： was playing
He was playing（play）computer games when suddenly there was a big noise outside
when引导的时间状语从句用过去式，主句表示过去正在进行的动作，所以用过去进行时。
was playing 过去进行时 ，很明显句子意思是当外面突然出现很大的吵闹声，他正在玩电脑
He was playing computer games.......
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