RF feedback causes squealing... which can be caused by the following problems:
Check the coax shielding connection inside the screw on UHF connector. Check for loose wiring inside the microphone and the connector. Check for bad ground on the antenna (shielding inside coax near antenna connection or loosed/oxidized braided ground wire on antenna bracket). A bad antenna ground will cause high SWRs...... causing the transmitted signal to reflect back in to the radio....
There are more..., but these are the most common problems.....