Dpbeats11

During v4 mode at highway speeds or cruising street speeds, under light acceleration a constant rumble/clanking will start from behind the passenger seat. Possible muffler area. The noise gets louder and speeds up when I increase the acceleration. The noise will continue until I let off the gas or increase speed to go back to v8 mode. I’ve had an aftermarket cat-back exhaust for about a year now, however the noise only started when the weather got cold. It will not make any noise at any acceleration while in v8 mode. This is only under v4. Taking it to dealer to have it looked at this weekend. Any suggestions of probable cause is greatly appreciated!