I just bought a 1986 K5 Blazer with a 305 as a plow truck. So far it's serving it's purpose just fine as I'm only using it to plow and drive 7 miles between my home and my barn. However, I've noticed an engine knock which increases with revving like when going up a steep hill. The truck has 154k miles on it. Since there aren't really gauges in it (just lights for engine temp, oil pressure, etc) I can't definitely say what the oil pressure is (the light has never come on). I'm just wondering what the most common issues might be? Where should I start? Plugs? Wires? Timing?

Any help would be appreciated.