Pontiac Repair: 2001 buick custom engine knock, piston slap, rattle sound

Question
Hello Todd, maybe you can help me. I have a 2001 buick custom, 3.1 v6 which just started knocking all of a sudden the other day. It's a solid thump or bang sound compared to a lifter rattle sound but is inconsistent in its rhythm and does not correspond with engine rpm. It will make this noise mostly say if the car is at idle and you give it gas. You put it in gear and hold the brake and accelerate at the same time and there is no noise. I had it looked at and was told it's a rod knock, but my experience with rod knock has been that it's always there and consistent with engine rpm. This one isn't. The shop was originally looking for carbon build up on the pistons, but that wasn't it. I cant exactly isolate the sound as it sounds like its here and then its there, possibly from under the intake manifold area, or or not. could it be something in the valve train? Thanks for your help, Chris

Answer
Hi Chris;

does the noise go away or quiet down after it warms up? if so it's piston slap and all of the 31/3.4 liter engines do it once they get high milage. If the noise is there all the time and it doesn't quiet down with tempature it's more serious. And the engine will need to be taken apart. if it's not at 1/2 the engine spped then it's not valve train related if it's at engine spped or 2X the engine speed it's a rod knock and that is serious and needs to be adressed. Di you try running it with the serpantine belt off to make sure it's not anything related to whast the belt runs? Good luck :0