The noise comes from the fact that the phone charger dc- (out the USB) and the audio common are both connected to the vehicle chassis. The charger uses a buck converter to drop the 12 V dc to 5 V dc for the USB and this involves a high frequency oscillator.

I thought about a ground loop isolator but not sure that would solve the problem as I am not sure it would block the noise from the phone charger.

They do make Bluetooth to Aux-out adapters. They are everywhere on Amazon and eBay. I found one on eBay for $12 that had left/right/ground on a 3 pin plug for some vehicle application. I just cut that plug off and wired it to the aux input on my head unit on my 2008 E320. You do have to give it 12 V dc so just tie it into the switched 12 V dc lead on your stereo or lighter socket.

I was going to do a Bluetooth input on my 87 300TD before I sold it. It has the original 780 Becker that I had modified to add the aux input. I was going to add a relay that was driven off the antenna switch to change it between radio and Bluetooth. The Becker mod intercepts the audio path when the radio is in 'FM' mode and puts it out to a 3.5 mm jack with switch contacts. So if the antenna is sent to the down position I was going to set it up so that it switches the relay so the Bluetooth receiver is in the audio path. And then when the antenna is put back up the audio path switches back to the FM tuner.

The antenna system on the 240D is similar with the dash mounted switch so you should be able to do this here as well.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech