I have a 98' sierra 240km on it, I have been having this ticking sound problem for about 4 or 5 months now. When i am going up a hill around 80 to 90kmh is when i can notice it the most. At first i thought it was rod knock, but when i lsitened closely it sounded like a tick more than a knock (thank god). Anyone have an idea what this may be? Some forums suggest its a bad lifter or valve. If it is something major i am going to rebuild!!