Got an 85 GMC K1500, 305 from the factory, swapped rebuilt 350 in it currently, it's developed a light knock! On start-up it's not there, after about 10 minutes of driving it's really noticeable (audible and feel it under your feet in the driver's seat) when braking. Now it could be that it's always there and the noise of the engine and exhaust drown it out, but you can only feel it when braking. Any ideas? Hoping it's some kind of detonation issue. Going to be changing out the spark plugs this weekend, throw some oil thickening agent in the block, and try a slightly higher octane gas. Any other suggestions would be much appreciated!