Drizzle the water in SLOWLY with the engine at a fast idle, 2,000 RPM or so. The water breaks up into a mist as it hits the carburetor venturi.
Before you try the water trick, try to isolate the knock to a particular cylinder by disconnecting spark plug wires one at a time. A cracked piston skirt or a loose wrist pin bushing will quiet down with the ignition disabled, and a rod bearing knock will sometimes change to a double-bump sound without a firing load on the piston. If you can change the noise by disabling a cylinder, better do a teardown to find the source of the problem.
The GMC V-6's are TRUCK engines, not similar to Chevy V-8's at all. They have a 60-degree angle between the cylinder banks, and fire on a 120-degree rotation sequence just like an inline six. They are low-speed, high torque engines, and they burn gas like it's going out of style. Don't even think about getting good fuel economy, but they really excel at pulling heavy loads for a long time.
Jerry
