Subwoofer making a loud humming

[Krellogg]

When my friend starts his vehicle and the stereo and sub turns on, the subwoofer makes a loud humming bass-like noise.

The same sub and amp was previously connected in my car, and it worked fine. He tried connecting it all in his 4runner with a pioneer head unit and its not working.

Its probably a really basic problem that hes just overlooking but hes lazy to take it in.

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[loopkiller 10+ year member]

Check the grounds and the gains.

 

[Krellogg]

fixed, something with the head unit