If it's an old style one ( pretty big box) that's what they do. I'm surprised you didn't hear it before.
The new ones are silent.
I put in a Progressive PD9245C with charge wizard, bought it on ebay for a great price, don't even know it's there.
Have checked the battery
water 3 times since installing back in May. The latest was today ( no water needed yet!)
It's doing a great job without killing the battery
( something my old one didn't do)
By the way when I'm home it's plugged into shore power 7/24.
Probably the best thing I've bought yet.