I have a 2004 Silverado 5.3L with 154,000 that has 2 maybe 3 problems and wonder if they could possibly be related to a single cause. 1. I have an intermittent loss of oil pressure. The pressure as one would expect is dependent on the rpm when rpm is 1000 or greater there is no engine knock when the rpm drops to under 700 the pressure drops to the point where the engine starts to knock. The problem self corrects lasting about 8000miles (2 oil changes) only to return. This has happened 3 times now. 2. There is a cold start tapping that lasts from a few seconds to 2 or 3 mins. 3. The antifreeze disappears at the rate of 1 gal per 3000 miles. There are no apparent leaks, oil appears normal, and no noticeable exhaust changes. Other than these issues the truck runs great getting about 18 miles per gal. Any suggestion?