First I am going to assume that the Building Entrance Terminals (BET) include primary Protection at both ends. You do not indicate what type of ports these are, analog or digital.
Second I am going to assume that your overall length of the cables does not exceed 5.5km, if it does then the inclusion of loading coils in the design would have been required to counter the effect of capacitance buildup in loop.
Poping noise is typical for a cable discharging it's natural capacitance charge or an indication of stray voltages. It may also be due to water getting into the cable or splice.
I would double check the grounding of the BETs and insure that the aluminum shielding is bonded to the same ground terminal using the proper "bullet bond" clips. If the OSP cable was spliced at any location, insure the aluminum shieldings are bonded together and that the splice is water tight.
If your ports are digital, consider using a different type of primary protector module rather then the standard "Black" used for analog circuit. Be sure to read the specifications for voltage ratings. If unsure, contact the manufacturer of your voice equipment for these specifications and their recommendations in the selection of a protector module to use in the BET.
Joseph Golan, RCDD