Popping & Spitting
An orginal Quad ESL may emit small "pops" and "spits" with or without music playing, for a couple of common reasons, one of which will cost you a lot of money...

Popping & Spitting? Colloquialisms used to describe very short, sharp transients occurring independently of musical signal.  These may occur while music is playing but are most commonly reported when the system is otherwise expected to be silent - that's why they are noticed!

Background:

A "pop" or a "spit" is caused by a discharge from the diaphragm to the stator, usually. It can also be caused by a stator to stator discharge - that would be bad, very bad,

 

Scenario 1:  You are running the speaker at too high a level. You do not have the protection board fitted to each speaker. When the input to the audio transformer reaches around 33 volts or greater, a discharge of a few thousand volts may occur from the back stator to the front stator, punching a hole through the diaphragm.  This creates a little "burn spot", which, being semi-conductive carbon, makes the next discharge even more likely. Keep doing this and you will burn a substantial hole in the diaphragm and the stator(s) as well!! A slight ozone smell may be apparent if this is your problem, and you wil see the spark when the music reaches high SPLs. The only real "fix" is to repair the diaphragm(s) and stator(s) - not cheap.  Fit protection boards after you fix the problem!!

Scenario 2: You are not running the speaker, but little "pops" are heard. These will be caused by small high voltage discharges to ground/earth, somewhere in the speaker. As they are very short-lived, it is quite hard ot track the cause down.  However, observing in a dark room you may see a little spark around the edges of a bass panel somewhere. Listen carefully and you will be able to "zoom in" on the location in all probability. The discharge can often be from the edge of a bass panel to one of the screws holding the side "cheeks" on the speaker. Look around there first, or remove the screws for testing purposes. If the pop goes away, you've found your culprit.