Booster air sound coming from depressed brake pedal

Hello,

Hard brake pedal typically points to an issue with brake booster.
This can happen if the vacuum hose is disconnected/cracked, for the booster. Also, for internal failure.

Inspect the vacuum hose for the brake booster. Make sure it is secured and does not have any dry rotting or cracks.
Also make sure the check valve is functioning. If the valve is clogged or stuck closed it will not allow the hose to perform as designed.

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.