Posted: Jun 16 2009 at 11:52am | IP Logged

Hmm... wow that's a tricky one.  My best guess is the piston slapping the valve, I.E. a valve return spring is broken.  If you reving the engine fast enough you'll get what is called valve floatation.  That's when the valves are going so fast that the springs don't actually pull them back closed before the cam lobe comes back around.  That's usually when the piston hits the valve still sitting open and destorys everything.  At best it bends the valve allowing you to limp off the road.  It's definitley something you want fixed ASAP as whatever is wrong will get worse and could possibly end up sticking out of your motor. 

An alternator won't make that kind of noise, whining and smoke yeah, but not a knocking sound even if it did have a bad bearing.

There is a neat little tool that can help.  It's like a doctor's stethoscope, but with a needle point instead of a flat disc.  We use this to detect exactly what area a noise is coming from inside the motor. It pinpoints an area really well.  Easy to use as the location with the loudest noise generaly gives you what is wrong.  High up on the valve cover means the valve train, near the oil pan is usually a crankshaft bearing.  Usually a parts store will have one hand. 

http://www.northerntool.com/webapp/wcs/stores/servlet/produc t_6970_385487_385487