I will try your very helpful suggestions, Dear Peter. At first checking ground of the areal and then making a direkt connection to the battery feeding the radio.
And I will report about it (in a new thread), so possibly others can benefit of the research.
I'm wondering if other 4/4 owner have a clear radio receive - to learn if it is an individual fault or a common issue.
Just try AM radio with engine out and ignition on (loud noise in my case) vs. engine out plus ignition out but radio keeps running in other key position (clear receive in my case).