If the mic is amplified and has a battery, then turn the mic down too. There should be a vol pot on the mic itself. NEVER turn it up more than half way. Better still, one third of the way up is usually the best setting - IMO.Also if it's an amplified one, replace the battery if it has one. And also try lowering the mic Gain on your radio as well.
Also, have you changed the mic plug from say a 5 pin to 4 pin? If so, make sure you have everything wired correctly. Hope you get it resolved and have a great day.