The heater in car is making a whirling sound only when the heat is on low and medium air flow

Sounds to me that you might have a leak in the line. Those heaters are all Vacume actuated.

sounds like youre water pump is shot.it mite be a cloged raidiator with low flow.or the worst of all a bad head gasket

All kinds of things to look at.
Start with coolant level in radiator. Very often if the radiator is low, the heater will not work, or work well, because if there is air in the system, air will rise to the top, leaving the heater core with an air bubble.
Whenever you replace any hose on the car, you must run the engine with the heater on at a medium high RPM (1500 RPM) for a while (5-10minutes) to "burp" the air from the system.
You'll need to make sure the system is full of coolant after that.
Then there could be a partly plugged radiator, a pinched or crimped heater hose, or the heater control might not be allowing the hot water to flow fast.
Be sure to test the temperature of the radiator, to be certain the thermostat is keeping the radiator coolant at near 200 degrees. A thermometer in the top of the radiator will tell you.
If you have a sealed system, you might VERY carefully feel all the hoses to determine the heat inside the radiator after the motor has been running for 10-15 minutes.
Thermostats can get stuck in the "open" position, requiring replacement before you will get heater heat.

Has the vehicle been run with anti-freeze its whole life? If not, then flush heater matrix.

With vehicle in cold state, disconnect heater matrix feed and return pipes from within engin bay. Turn the vehicle's internal temperature setting to hot. If the vehicle is fitted with climate control, then do this with the ignition in the 'on' position.

Take a garden hose and with low pressure first, try flushing the matrix. If the water pressure is not enough to flow through matrix at low to medium pressure, then a blockage is almost a certainty. If water does not flow at first, then try using a high pressure air-line to force air and any blockage material through the matrix and out of the exit pipe. You can also reverse the flow to blow or flush any dirt out of the other end.

Once all is clean and free flowing, reconnect and fill entire system with a 50-50 anti freeze-water solution.

If blockage cannot be resolved, then replace heater matrix.

If no blockage is present then continue to check heater control switches and wires.

Check low antifreeze level--- Replace thermostat-- heater coil plugged, check for flow through heater coil, heater coil outside plugged no air flow replace air filter in dash and clean outside of heater coil if you have very low air movement to defrosters--- try this last thing first --- cheapest things first ,

No hot air is due to low coolant, or even poor coolant flow. The engine heats up the coolant, and it passed through your heater core, and the heater blower motor pushes air through the heater core to warm up the air. If your "fan"is working, but the air is cold, make sure your coolant level is good.

Sounds like the fan will not run, and the air pressure from moving quickly is the only air running through the heater core. Check all the fuses in the fuse block for any open (blown) fuses. If there are no open fuses, have your mechanic check for vacuum leaks, as they may be inhibiting the motion of the louvers that re-direct air in your heating system. (That is, if the fan DOES run and there is still no heat without moving quickly.)

The heater core may be coroded and pluged up. Its common on older cars. f the cooling system was drained recently and refilled its possible there os air in the system which will block the flow through the heater core. You should also check the valve ,usually on heater hoses to make sure it is opening and closing properly when you turn the temp up.

sounds like it might be low on coolant. Low coolant levels will stop the flow and circulation to the heater core, and also might be the cause of overheating

What kind of van is it? Sounds like the blower motor resistor.