Coolant Leak

1999 Toyota Camry LE 116,000 miles 4cyl: I am leaking engine coolant but when I took the car to our mechanic he was unable to locate any leak. However, the car makes a "slurping sound" like the coolant is being sucked into the engine while I'm driving. Also, the heater in the car works sporatically and when it is on sometimes you can smell the coolant. There is no noticeable leaks but it needs a refill almost everyday. If you have any ideas on what the problem could be that would help. Also, 3 years ago replaced the head gasket, water pump and timing belt if this makes any difference.