Make sure that the antenna base is well grounded. Disconnect the antenna, then start the engine. If the noise is gone, you will know that the noise was traveling through the air. If the noise is still there, then it is coming through the wires.
The magneto shield is grounded at both ends, however the shield is not continuous from end to end, being separated at the center conductor splice on the forward side of the firewall. Make sure that the shield is NOT spliced.
Check the voltage regulator to make sure it is well grounded (not mounted on a painted surface). And check the battery ground wire where it is attached to the airframe (no paint).
Let us know if you find the noise source.
Joe Gores