If you open a can, and the can opener doesn't open it on the 1st try...........?

metal should not be present in food. The noise is from hitting the previously cut spot so no metal to resist the blade and it is just cutting air. The food cans are tin plate and not aluminum some cans are coated inside if they contain acid foods such as tomatoes. Tin and zinc cans with lead solder have not been used for some time. Wipe off surface before opening and no materials should be introduced into the food. If opener slips often it is time to replace they all have a a like expectancy.