Why does it sound like a head gasket is broke?

Why does it sound like a head gasket is broke? I have a 2001 ford escape and we recently changed the head gaskets and looks for leaks in the intake as well as their is no oil and water mixed the oil still looks fine but it sounds like it has a head gasket or a leak but I haven't been able to figure out what could make it do that. Is there anything else that could possibly make it sound like a tractor? Any help is appreciated.