thumping sound from subs

I have stumbled across the weirdest problem with my subs. They have started making a thumping sound when I turn on the HU. Not a turn-on thump. After the amp powers up, they will make this sound several times and then stop doing it. I have tried replacing the fuses, checking all the connections on the amp, checking the ground. Since both amps turn on with the same wire, I have elimanated the HU as the problem. Only the bass amp is doing this. Could it be something inside the amp going bad? I've never ran into this problem before.'85 Toy

Could be a level mismatch in your gains, or a bad ground (or a ground that has changed or corroded) or a damaged RCA input, or yes a sign of early amplifier failure.  Take the amp out and have it bench tested.Support the12volt.com

Are both amps grounded at the same point?

Yes, both are.'85 Toy

I have still been looking with no luck. Could it be one of the subs? I am going to replace the amp and see if it is the problem. '85 Toy

Like I said, pull the amp, take it to a shop, pay them $20 to have it bench tested.Support the12volt.com

I had this problem occur when I had two 15's in an eagle talon 2 years ago, the shop I went to said my head unit was not putting enough volts out of the rca output.  They sold me an electronic equalizer and everything worked perfect.  They said this happens often when the amps gain is turned up and the preamp output is not very strong.  Just a suggestion, this may or may not be the same exact problem you are running into. 

Just to update my problem : Seems it was my amp. The shop can't actually find the problem, but I hooked up an old Orion 225HCCA that I had sitting around while they were checking the Crossfire amp and the "thumping sounds" I was getting went away. Thanks for all the info 12volter's.'85 Toy