13-inch iPad Air (2024) - internal microphone background noise

Purchasing a used device from anywhere other than Apple is potentially risky. Whereas Apple's renewed/refurbished devices are guaranteed to perform as new - and are backed by an Apple warranty - previously used items purchased from elsewhere do not carry the same assurance.

Given that you have a hardware issue that cannot be diagnosed remotely, you would be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - and you can explore your available options:

Genius Bar Reservation and Apple Support Options - Apple

If the iPad has a hardware fault, as is perhaps very likely, the cost of a repair may be uneconomic. For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange a qualifying iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. An out-of-warranty repair while expensive will be somewhat less than the cost of a replacement new iPad.

As for out-of-warranty repair costs, Apple offers a fixed-price price repair. You can verify eligibility and cost for your region/country here:

iPad Repair & Service - Apple Support

An iPad's internal microphone should not be inherently noisy. If the purchase can be cancelled, you might be best advised to return the item - and instead purchase a renewed or new device directly from Apple.

If sourced from Apple, you will have benefit of a device warranty. The iPad will have a new battery and outer casing - along with assurance that the iPad performs to specifications.