Could be part of auto de-frost in freezer, it's normal. On that subject, check fridge still working. A few years ago there was a loud pop and the cooling element had broken although there was vapour at the same time.
Last time that happened to me it was when I had a microwave/oven type one that have two different settings for use and I ended up putting the foil in the microwaveable!!
Big bang, sparks the lot!