Could be the speaker also going out. Take and have it frequency tested. If bad replace it. I replaced the 10inch in my R40 with a Richter 10 from Seismic Audio. Three inch voice coil, aluminum frame and 200 watts for $69.First, set all controls flat, and turn off all voicings, and see if the problem goes away. If not, check your cable and all connections. Are you using an active bass? If you are, check your battery. If the problem persists and the distortion is showing up intermittently, I would contact a Fender authorized service center, and have them look at it. I’ve had a similar problem with my Rumble 40.