Whisper is a format which stores timeseries data for graphite (software for storing timeseries, normally for system monitoring).
There is one whisper file for each metric recorded. The file is a fixed-size file, in a similar way to RRD files.
While there is no magic for the file, the first four bytes is "aggregation type": a value between
00 00 00 01 and
00 00 00 05.