I have a 181 Mercruiser 4 cylinder (3.0 liters) in an old truck, a 1967 Landrover. It is a rebuilt engine with less than 25 miles on it. All of a sudden it has developed what I believe is a valve noise. Using an automotive stethascope (sp?) it seems that the noise is coming from the valve (3rd one back) or where the rocker arm contacts the valve stem. I pulled the side cover and the valve cover and there are no obviously bent pushrods or valves, everything looks ok. This engine is a variant of the GM 2.5, although I am confused about which engine it is exactly. I was told it is the same block as a late 70s 2.5 Chevy Monza. Is there a way to adjust the valve lash on these engines? I am not used to hydraulic lifters so it's all new to me. Anyone with any ideas on what might be causing the noise? Thanks in advance for any suggestions.