Start by doing some maintenance on the antenna system. Disassemble the mounting bracket and clean everything thoroughly. Check the coax, and if it looks a bit rough, replace it. Make sure the connections, at ALL points, are tight, especially the little socket where the PL259 "tit" slides in. Once done, check the antenna tuning with a good meter, or better yet, an analyzer with a frequency readout. Once all that is done, see if the problem is still there. If so, try running a wire from a radio case screw to the main ground. This could cause ground loop issues (picking up engine whine in the radio), so use it as a last resort. If all else fails, stick a radio shack part # 273-105 on the mic cord just under the mic head, like in the middle of the first couple coils.