Liquid Nitrogen

Albatross

i was doing an experiment with liquid nitrogen, i had to put an aluminium body initially at room temperature inside a glass with the nitrogen at 77K, first it starts making a loud noise then a lower noise that is slowly increasing and finally near the equilibrium point the noise gets strangley very very loud almost instantaneously and then stops in about a second. Maybe it's something stupid, i found it as a strange behaviour and i can't explain it, someone told me that maybe the lattice of aluminium changes at that temperature, but i don't like it, i find strange that the intensity increases and then near equilibrium instantaneously increases a lot. can someone explain that? thanks

my english is very bad, i'm sorry for that =P