Regular click trains at a rate of less than about 40 Hz sound like individual regular events, perhaps a bit like machine-gun fire. Click trains with rates faster than about 40 Hz merge into a continuous "buzz", where the pitch of the buzz depends on the click rate: the faster the rate, the higher the pitch.
The two sound examples below illustrates this. The first example consists of a click train, where the rate of the clicks doubles every 3 seconds. During the first 3 seconds, the rate of the clicks is about 10.7 Hz, then it increases to 21.4 Hz, 43 Hz, 86 Hz, 172 Hz, 344 Hz, 689 Hz, 1378 Hz, 2756 Hz, and 5512 Hz. Clear pitch emerges between 43 Hz and 86 Hz, although at 86 Hz there is still 'flutter', and full smoothness of the resulting percept occurs only at rates of a few hundreds Hz.
The second example consists of clicks which are presented initially at a rate of about 10 Hz. The rate then continuously speeds up, to a rate of 10,000 Hz, and then slows down again. At the beginning and the end you will hear a rapid train of individual clicks, but in the middle you hear a buzz of rapidly rising, then falling, pitch. The sequence is illustrated in the figure below.