Is the snow and ice a result of climate (it has snowed recently), or is the heat pump creating it?
Regardless, you will need to clean it off. The reason your outside pump sounds louder is because the air is not moving through he unit like it should. Itís just like a box fan when you put it up against a wall. The air stops moving past the blades, so it spins around with the blades, making the fan louder. This is very important because right now your unit is not working efficiently, and could overheat. Your inside unit will sound normal, BUT, since the outside unit is not getting any air through it, your auxiliary heat comes on, which is nothing more than toaster coils that are very expensive to run.
Your heat pump works in the reverse of an air conditioner. A/C units extract the heat from inside the home and exhaust it outside, whereas your heat pump extracts the heat from outside, and releases it inside your home. I know it sounds funny to think that the unit can find any heat in the outside air during winter, but heat pumps are pretty good down to about 35 degrees. Below that, and one or more of the aforementioned toaster coils come on. When all of them are on, the wheel on your electric meter will be spinning at about 27,000 rpm. This is why it is important to keep the outside unit as free from obstructions as possible.
Now, heat pumps will ice up in certain climate conditions, even without the presence of snow. The moisture in the air simply freezes when it comes in contact with the coils. On its own, it (the outside unit) will become a frozen block of ice, except for a sensor on the unit which detects the freeze, and goes into sort of a defrost mode, just like your fridge. When that sensor goes out (it went out on me 3 times in 6 years) the unit will not defrost. Usually there is a flashing light on the thermostat, or a code showing up on a display, but in my case twice there was no code because the sensor thought it was 90 degrees outside (it was 24).
If you have a lot of snow on the unit, you will need to clear it away. The defrost cycle may not be able to handle all that extra snow. If, after you cleared it away, there is still ice buildup after a couple of hours, call in service. Something is wrong