Very loud low to high sweeping tone only with engine running

[Johnny_W]

I've installed a RetroSound Longbeach stereo in my 1967 MGB GT.  With the engine off I have no issues.  With the engine running the receiver sounds great for 3 to 5 minutes then I get a very loud low to high sweeping tone that may repeat or change to clicking.  If I switch to another input (bluetooth, aux or usb) everything is fine.  The issue only effects the tuner.  If I turn the radio off and then on again the tuner works for 3 to 5 minutes and then I get the tone again.  The Sony that the car came with did not exhibit these symptoms but got poor FM reception, looked wrong for the car, and was missing many features the the RetroSound radio has.  I have already returned one RetroSound radio, the video below is the second radio doing the same thing.

Here is a link to my stereo making the noise (The smoke is from me, not the stereo or the car): 



Tests completed so far:

1) I've replaced the generator and external voltage regulator with an alternator with internal voltage regulator.  (No change)

2) I've run jumper cables from another car withe the other car's engine running. (No tone)

3) I've run the stereo from an external power supply, all grounds disconnected, antenna lying on the hood on a cloth, radio hanging on wires so no metal is touching the car. (No change)

4) Same as 3 but with the antenna disconnected (Static for 3 to 5 minutes then a loud clicking that drowns out the static)

This car does have the original ignition system (distributor, points, condenser, coil) if that helps.

I'm open to any reasonable suggestions.

Thanks

Johnny_W

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