Installed a new blower motor regulator (procupine) and the noise feedback into blower motor windings and am radio went away. Whatever causes this is relatively rare compared to the general regulator failer that causes the blower motor to not run. Interestly enough the new regulator is a bit different from the original one I took out. Same part number and same manufacturer. I did try some used ones but the only one that solved this noise was new procupine.