Rattle from engine when holding same speed

I have a Matrix with 150,000mi on it, and it runs just perfectly, but makes a rattle sound when I hold at any speed, in gear. It doesn’t rattle when idling in neutral, with clutch depressed, or accelerating decently. Only holding at a certain speed, AND it can do it when I start in first. Any ideas?

A loose exhaust heat shield can start to rattle at a certain frequency.

Tester

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