i took the panel off & yes it is a wfco. and there is a very slight buzz coming from the interior speakers also.
Ok, so now ALL the speakers are "buzzing". Is this with the entertainment device on but the sound turned down? If so, the buzz is probably a bad ground or ground loop from the entertainment device.
Please, post what you have for the trailer, AKA "Jensen DVD player w/radio" or such. If any of the connecting wires in the back of the device, even one strand of wire are touching each other, you will get some interference.
Do all the speakers buzz if shore power is removed?