If trying it on a new power adaptor does not help, you may need to have a
technician look at it.

I understand the audio is distorted on battery operation, too. On the other
hand, if both the adapter and the batteries were bad... Weak batteries will
give distorted audio.

I know that a even new adapter will give a slight hum on CW and SSB reception.
It also puts hum bars on FAX pix. Because the D cells don't last long, I built
a little filtered power supply for my DX-392.

I also don't trust wall adapters. A ham friend had his shack burned out due to
the failure of one of those. If they run warm when they are in use, they might
be okay. I had one for a video switch that ran too hot to touch.

Using house junction boxes, each with a socket and a switch, plus a very low
rated fuse, I felt safe in running a couple of units full time.

But for the 392 with bad audio even on FM, the PLL may be trying to unlock. The
AA cells may be weak.

Try the easy stuff first.

Bill, K5BY