Noise in 340...help

Please be more specific on the type of noise. Here are some suggested adjectives.

Loud rapping sound
Sharp double knock
Dull rythmic thumping
higher pitched tinny tapping noise

It would also help to know how the car is equipped, headers, 4 speed or auto, does the noise change when you push on the clutch or put it in gear? Can you pull a plug wire and make it stop? Does it get louder and/or faster, with RPM etc.?

Many times diagnosis is just a simple process of elimination. By isolating the things it is not you can deduce what it is.