Why is my car leaking a rusty coloured watery substance?

My car, a nissan pulsar 1995 is leaking out a rusty kind of liquid. I have no idea what it is. The night before my "check light" came on so I put oil and water in and then this morning I was driving to work and the "check" light was still on. And there was alot of leakage of the rusty liquid stuff. Also I noticed that sometimes i can hear a rattling under the bonnet when I turn the car off. Does anybody know whats wrong with my car please??