Everything M_C said with addition to go over every wire and connection with a fine tooth comb has chances are this is something simple like a loose connection or switch going down sending out noise.
Obvious but Check no high voltage cables run along side signal cables.

One last thing.!! . . . Have you changed any other equipment close by the machine that may be interfering.? I ounce had a compressor in the next room separated by concrete block wall affecting my machine when coming on.!! . . . . took me months to realise only happened when compressor fired up because it didn't always happen.! . . . Nightmare stuff.!!!