Alternator whine with a twist

[Rut Row]

I plug my Midland MTX-275 external speaker output into my aftermarket (Dual Elect XDM280BT) radio aux input plug. I get a noticeable alternator whine. I get the whine with the GMRS turned on or off. I tried a shielded 3.5mm audio cable and that didn't help. I do not get the whine if I plug the car aux into my cell phone's 3.5mm output. I do not get the whine if I unplug the cable from the GMRS.

I tried putting an Install Bay IBNF30 Noise Filter on the GMRS radio but that didn't change anything; still had the whine.

Has anyone had the issue and does anyone else plug the GMRS into the car radio?

 

[macleanflood]

How's the GMRS grounded? Straight to the battery or somewhere else?

-Mac

 

[Rut Row]

It is connected to the body at the ground near the brake master.

BUT! Yesterday a friend came by with a audio isolation transformer (https://www.amazon.com/dp/B06XQYN77L/?tag=wranglerorg-20) and that seems to have fixed it. Now to buy one for myself.

Thanks for responding!