Popping noise from eathquake amp.

So i have 2 Earthquake PH5000/d1 amps running 4 FI SSD 15s. I have found a low popping noise in the speakers from 1 of the amps. When i unplug the RCA cables from it it gets louder. I tried switching the 2 amps and it stays on that one amp. I also have an audiocontrol in dash epicenter that can measure dbs. One night after i built the box i went for a test drive i was hitting 158 on it. the next day and since then i have not been able to hit above 155. Today i was outside messing with it and I unhooked the speaker wires from the amp that is popping and the subs on the other amp got louder. That was while the truck was running and music playing. So i hooked all 4 to the one good amp and i was easily hitting 155 dbs. So i called Earthquake and im going to send it in for repairs. My question is does anyone know what inside the amp could be causing this?