2000 Chevy Suburban low coolant

Recently the "service engine soon" light and "low coolant" light went on. Checked the reservoir and it was half full so I added coolant. Also have noticed a sound like water gurgling and rushing at the top of the dashboard under the hood. The engine has not overheated or even gotten too hot, and there are no noticeable leaks on the ground after the car is parked. Have not seen smoke in exhaust and the coolant appears uncontaminated. Took it to the dealer, who said the head gasket was leaking and had to be replaced (at several thousand dollars!). Took it to a mechanic for a second opinion and he said it was a water pump leak and we should also replace the thermostat (he didn't actually check the gasket, but said there was no water in the oil or oil in the coolant). Any suggestions and advice would be greatly appreciated.