5.3 knocking sound

Hi Folks, Have a 2001 Tahoe Z71, with only 87K on it. On a trip up to Mt. Rainier the motor started making a "ticking" sound, and seemed to be there even after warm-up. It would go away, as well, here & there. But it easily made the 7 hour round-trip to the mountain & back. Now the noise is...