Engine noise MX-5

2003 Mazda MX-5 Miata with 25000 miles. Mysterious engine noise in 4th gear only, and only when accelerating between 3000-4000 RPMs, most noticeable when accelerating up hill ? kind of a faint tapping or rattling, which my mechanic is unfortunately unable to hear. Does not disappear when I switch to Premium gas. Only significant modification is a Racing Beat dual exhaust muffler.

A loose exhaust heat shield can cause a tapping/rattling noise under certain engine operating conditions.