I just bought a 90 Mustang LX 5.0 HO. It has a newly installed Vortec Charger on it and the rest of the car is completly stock. Its not what I would have done but it is what it is. The car is completly original. Engine has a lot of miles and the block has never been taken apart. The Charger is set up for 6lb maximum boost. Stock injectors as well as everthing else under the hood. The car passes CA smog with flying colors. It does not burn oil and the head gasket is good. They car starts up and runs great. It idles perfect but when the car hits between 2500 and 3000 RPMs there seems to be a knock or grinding noise. This happens everytime but only at the stated rpm. The guy I bought it from said it was confirmed to be a crank shaft going bad but I would think it would happen all the time if that were the case. It idles smooth and has planty of power if asked for it. Let me know what you think it could be...

 

What about above 2500 to 3000?

Stock injectors? as in 19lb's?

What octane fuel are you running in it?

 

You may be running low octane fuel. And it could be causing a valve rattle.

Or you could have a bad crank, due to a main bearing going bad.