Grand Am Horn blares constantly?

I have a 2005 Pontiac Grand Am SE. My horn wasn't working so I pulled off the horn and cleaned it up then replaced a blown fuse. The high tone horn worked fine but the low tone sounded muddled like it was under water. 4 hrs later at 5am my car horn goes off in a steady tone. It is not my alarm going off. My husband thought it sounded like the low tone horn. Anyways I pulled the fuse immediately and it went off. Any idea what causes this and how much money I'll have to sink in it to repair it? It was a tad bit below freezing this morning but probably just in the 20s.