strange high pitched electric beep coming from rear

Once in a while during hard take off or braking, I get a high pitched electric beep coming from the passenger side rear inside the car. Does anyone know what this is? I just bought the car so I do not know what modifications have been done but I do know there was a sub in the trunk at one point. Every time I try to record it it stops happening but will start again in a few days then stop again. It's definitely coming from inside the car. I just don't even know where to start with this and would appreciate any ideas. It's not the same beep as the speed warning.

No one else has experienced this? It sounds like it's coming from the passenger rear speaker grill and sounds almost like a piezo buzzer. It's happening more often and more randomly too now. Sometimes I'll be just idleing and it will come on. I've never experienced anything like this in another car I just bought my mk6 so I don't know much about the car. I'm going to try and take the panels off to get to where the speaker is I can't think of anything else to do

I think it has to do with speaker wiring. Maybe fold down your rear seats or lift up your trunk floor to see if you have some wiring for a sub.

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