most likely it's not ice. the fluid's viscosity increases as the temp decreases. what you probably hear is the hyd pump cavitating - the fluid won't flow fast enough from the reservoir through the suction side to the pump. the pump pulls, reducing the pressure & causes the fluid to 'boil' even at very low temperaature. as the fluid warms, the 'buzzing' noise goes away. run the engine at LOW rpm & allow the fluid time to warm up. GRADUALLY increase the rpm (over the period of 30 or so minutes) until the fluid warms.
If you're not sure when it was last changed however, I'd change it and the filter as a prophylactic measure. good luck sir