My propane tank is ticking / clicking?

i just got a grill in trade for an old lawn mower, the tank was empty and i just got it filled, but the tank is making a ticking sound , i hooked it up, and the sound went away after i opened the gad valve, and the fire burned good, what can it be, also, if its the regulator wich i think it could be, can i change it without losing the gas i just bought?? or can i just leave it like that untill i finish the gas? will it be safe?