Pre-echo
The faint smear of noise you can hear just before a sharp hit in compressed audio, where the codec has spread quantisation noise backwards in time.
Transform codecs such as MP3 and AAC work on blocks a couple of thousand samples long. Quantisation noise is spread evenly across the whole block, so if a block holds silence followed by a castanet click, some of that noise sits ahead of the click. After a loud event the ear masks noise for up to hundreds of milliseconds; before it, only for a few. Anything that leaks further back is audible.
Codecs fight it on several fronts. MP3 and AAC switch to short blocks when a transient is detected, MP3 keeps a bit reservoir to lend extra bits to difficult frames, and AAC adds temporal noise shaping. Castanets, harpsichord and glockenspiel are still the classic torture tests, because that is where a low-bitrate encoder gives itself away first.