Plate amp troubleshooting

I’m having an issue with a plate amp on a JBL ES250P 12” subwoofer. When I turn on my system the whole house shakes. I thought my wife was in the attic slamming a huge box on the rafters the first time I heard it. It goes away after 30 seconds or so and it only fires 3-5 times until it stops. It also doesn’t happen when a signal is going thru the pre-amp. I’ve swapped outputs from my receiver and replaced the coax cable to no avail, so I’m pretty confident the issue is in the plate amp. I was just wondering if anyone would have any input as to where the issue may be in the plate amp. Any suggestions would be appreciated.