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did you try only having the center pin of the rca connected to the radio? you can usually try this by just barely plugging it so the center pin makes contact and the ground shield doesn't.
if this fixes a ground loop isolator (rca filter) should work.

also where is the gain on the amp set, if you back it down does the noise get quieter?

i like to tune amps with the radio at about 3/4 volume then bring the gain of the amp up until you get the volume you want.