subwoofer making constant popping sound.

I hooked my sub up to my amp last night. Sub is producing a constant poping sound while the amp is on whether the volume is up or down. Equipment installed includes the following:

- Factory installed head unit- JL Cleansweep- JL MHD900/5 - 5 channel amp- JL Stealthbox- JL C2-570x front & rear speakers- 16 gauge speaker wire (Factory harness to Cleansweep then from amp back to the new speakers)- 4 gauge power cable (From battery to amp. Negative connect to truck frame. Metal was sanded before making connections)- Monster rca cables (From cleansweep to amp)Cleansweep was calibrated after install. Front & Rear speakers output clean & clear sound at all volumes.

Amp is set on 4-channel (can be set to 2 or 6) since there are only 4 channels coming out of the Cleansweep.

My thought is that I seeing possibly a voltage drop but I really have the clue and would appreciate any thoughts. Like I said the amp seems to work great with the front and rear but receiving a popping sound (pop pop pop) from the sub when connected.

One last thing.. I did connect the sub into the rear output of the amp to check to see if it was a bad sub and I get clean sound out of it.