Sub rumbles when engine revs or decclerates

[deanproxy]

This is in a 2016 Ford C Max Hybrid.

The Amp and sub are a JL Audio ACP108LG (8" subwoofer with built in 250 watt amp)

I'm tapping into the rear speakers and using the amps High-Level inputs. I am running the power cable down the drivers side. The left rear speaker input cable is running across the back seat (under the carpet) and meets up with the right sides speaker input cable to use as the high-level inputs. I'm doing this to avoid as much contact with the power wire. The ground is bolted down to the rear chassis on a bare metal spot.

When the cars gas engine kicks in, the sub makes a very loud wave like noise. This happens at a consistent volume even if the head unit volume is turned all the way down. When the car decclerates (again, in gas engine mode), the sub rumbles loudly. Once the electric motor kicks on, the sub performs as expected. It's only when the gas engine is in use.

Is this a ground loop issue? I am tapped into the rear speakers which are amplified by a factory amplifier, so I was thinking this may be an issue. The ground to the new amp is solid and to bare metal, but it's grounded near the trunk while the factory amp is grounded at the front of the vehicle somewhere (the factory amp is behind the glovebox)

Any suggestions for fixing this issue?

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[deanproxy]

Problem found and solved.

It was the active noise cancellation. When the motor kicked on, the noise cancellation would kick on as well. I pulled the roof liner and pulled the wires out of the 3 mics and taped them off. There may have been a more elegant way to fix this, but I didn't find one after googling around for a few hours.

Nevertheless, the sub no longer rumbles during acceleration.