I have two sump pumps in my basement, one on the front wall and one at the back corner. These came with the house and are both ~15 years old. The one in the front runs constantly whenever there is precipitation or melting snow. We know this because it is right under our living room and it's very loud when it turns on, and when it's done pumping it ends with a loud clank!
I'd never heard the back one run before - I've always thought we had good drainage at the back of the house since it goes downhill. Today I dumped 10 gallons of water into the back sump just to make sure it worked. Man was I surprised to hear how QUIET it was! I was standing right over it and could barely hear it run. When it was done pumping there was a slight 'click' and that was it.
Which makes me wonder... why is the front one so loud? I'm no plumber but my first conclusion is that it's old and starting to fail. Am I on track? Should I make plans to replace it, or is it normal for these to get louder as they age? If our front sump stops working we're in big trouble as our basement would flood pretty fast.