What could make a spring or boing like noise under hood that resonates?

A spring or boing-like noise under the hood could be caused by a loose or worn serpentine belt that vibrates against pulleys. Additionally, a faulty tensioner or idler pulley might not maintain proper tension, leading to a resonating sound. Other possibilities include a loose heat shield or exhaust component, which can also produce similar noises when the engine runs. It's advisable to have the vehicle inspected to identify and resolve the issue.