I recently found a strange problem on a 9090 or 9090db not quite sure, but could be your issue if the noise is a hum rather than a static noise. On the underside of the unit, you can see the main power supply board in the center back. You need to remove the bottom cover to see. One of the mounting screws is used as a main Ground point for other boards including the preamp and tone amps. The screw is supposed to grounf that point of the circuit board to the chassis, and then they had soldered the top of the screw to the foil trace on the board. The problem I found was that the screw had loosened a bit ober time, probably from oxidation, but you couldn't tell it was loose to the chassis because it was soldered to the board. I had to desolder to screw from the board, and then tighten the screw, and resolder the head to the board trace.
You will be able to tell if this is your issue by removing the bottom cover and flex the chassis a little to see if the hum comes or goes away. Then you can get a piece of wire with an alligator clip on one side, or just screw down onto the ground terminal on the back used for TT ground. Then get the other end of the wire and touch it to that screw underneath in the corner of the board, that has a lot of small black wires soldered to the board traces right near it. If the screw is not tight to the chassis, doing this will stop the hum, letting you know you will have to unsolder the screw head, tighten the screw, and then resolder the screw head to the board
