Here's an old diagnostic trick- - - -try retarding the ignition timing about 10 degrees- - - - -so the spark happens when the BB is about 1/2" to the right of the pointer, toward the inner edge of the timing hole toward the pushrod cover. If the pistons happen to be installed backwards, the noise will diminish, or maybe go away entirely. Performance will be lousy due to the late timing, but there should be a noticeable difference in the noise.
Jerry
