Noise of microwave exhaust fan

Question

I have a new above the cook-top microwave (GE Model JVM1540) which is vented to the outside through a top exhaust vertical duct. The exhaust fan is loud and there is practically no difference in the noise produced by the high and low exhaust speeds. I have wrapped the duct with insulation in an unsuccessful attempt to lower the noise of the fan. Is there anything else I can do to lower the noise produced by the exhaust fan?