Be careful playing with the blower speed. If you slow it too much, you may end up with an iced indoor evaporator coil. Here is what I suggest. First I will assume the installer put together an heat/cool combination that's close in sizing. So if you go to the outdoor condenser unit and read the BTU rating you can determine what minimum flow rate of air you'll need. For every 12,000 Btu/hr (1 ton) of condenser cooling you'll need approximately 400 cubic feet per min (cfm) of air flow through the indoor coil. In the blower compartment of the air handler you should find the cfm for each of the available speeds. These cfm values will be according to static pressure. For now use the cfm values for 0.1
Now that you know if you can adjust your motor speed, that may be enough. However, if, as I suspect, the motor is already set to the proper speed (a good installation), then you go to Plan B. That involves mitigating all noise sources from the duct system. You have 3 usual sources. First is fast air flow into return registers or out of supply registers. This whistling noise is like wind blowing in an open car window. If that is the problem, make sure all return and supply registers are fully open. If they all are, then some duct modification may be your best alternative... increase the physical volume the air flows through (bigger/more/ducting.
The 2nd noise is from air handler vibration transmitted through the physical duct. If they are metal ducts and you can hear rattling or vibrations, then this may be the cause of your noise. Easiest way to fix that is insert a piece of flexduct at each supply and return connection at or close to the airhandler. This will stop the vibrations at the airhandler from propagating along the duct.
Last noise is the same vibrations but conducted by the air in the duct. Here, I suggest introducing a gentle bend (S curve) in the duct using flex duct. You want it gentle, no sharp 90 degree bends, that will restrict airflow.
Since the last 2 sources involve vibrations at the airhandler, I would start there first. Put your hand over the blower compartment when the blower is running. If you feel excessive vibrations it is possible your blower wheel may be out of balance. They can throw a balance weight as they age. Also consider shock absorbant pads under the airhandler. Any way to decrease the vibration at the airhandler will mean less noise for you.