My wife's 1997 Bravada with 80k miles has had a lifter rattle for a couple of months now. The only time in the past that I noticed this was comming up an approx. 30% grade hill by our home. Lately seems to be getting worse IE. under moderate acceleration lifter rattle can be heard. Replaced the O2 sensor a few months ago, other than that...I'm at a loss. "BOWTIEBANDIT"....GOT ANY IDEAS???