Difficult to say but probably slugs. Use a toilet paper roll or tin can with top and bottom out and put around the plant as a collar.

As long as the plants are growing in spite of the chomping, don't worry. Beans can take a lot of damage and still produce a crop. Slugs can be a problem though. The way to find out is to go out about 10 pm with a flashlight. If you see slugs feasting on the beans and you don't mind handling them, dump them in a bowl of soapy water. Do this for a few nights in a row and you will cut way down on the slug population.

Alternatively, you can plant more (or plant twice as many - some for you and some for the slugs). My first bean planting this spring took a lot of slugchomps so I filled in with more seeds and everything is now growing well. I think I planted a little early and the plants were not very strong, so the slugs took advantage of them. Nevertheless, many of them survived to grow well and the ones that got a lot of chomping didn't grow much but they are flowering all the same.
-O