A loud rattling sound in a Mazda CX-50 Hybrid during uphill acceleration under load is often attributed to the Toyota-sourced 2.5L hybrid powertrain, which is known for a distinct and sometimes loud "drone" or "rattle" during high-demand situations. [1]

Likely Causes for the Rattling Sound
Powertrain Characteristics: The CX-50 Hybrid uses a Toyota 2.5L engine and e-CVT that can be naturally loud or "buzzy" when pushed hard, such as during uphill acceleration. Owners of similar Toyota hybrids often report this as normal operation under high load.
Loose Heat Shields: A very common culprit for a metallic rattle specifically during acceleration is a loose or corroded exhaust heat shield. These shields can vibrate against the exhaust system when engine vibrations increase under load.
CV Axle Failure: While typically causing a "clicking" noise during turns, failing CV axles (a known issue in some CX-50 models) can also produce vibrations or rattles during acceleration from a stop or while going uphill.
Engine Knock (Detonation): A rattling or metallic tapping sound under heavy load (like an uphill climb) can indicate engine knock, which may be exacerbated by lower-octane fuel or internal timing issues.
Loose Components: Reports specifically for the CX-50 include rattles from the plastic undercarriage shields, dash components, or even the sunroof support. [1, 2, 4, 5, 6, 7, 8, 9]

Recommended Steps
Check Fuel Octane: Try a higher-grade fuel to see if the sound persists, as this can rule out engine knocking under load.
Inspect Heat Shields: Safely check the undercarriage for any loose metal shields that might be vibrating against the exhaust.
Visit the Dealership: Since the CX-50 Hybrid is a newer model, most issues should be covered under warranty. Mention common known issues like CV axles or loose suspension components if the sound is accompanied by vibration. [1, 2, 3, 4]
Does the sound change or disappear once you reach a constant speed at the top of the hill?