Sounds like an amp issue. Had some add-on amps with bad solder joints do the same thing back in my basshead days.
Not sure if the amp in these cars is in the headunit or not. I'd take it to the dealer for a warranty fix.
2013 Focus SE 5M Daily Driver
2003 Mustang GT V8/Auto Wife's Daily
1986 Mustang GT V8/5M Toy
- Jody -