Click noise when AUAudioUnit.startHardware()

Testing on real device iPhone 6s, iOS 15, headphones, Xcode Version 13.1 (13A1030d). Also click noise when audioUnit.stopHardware(). Clicking even if the callback buffer is filled with zeros or AudioCallback = {}. Clicking when init() and also if called at runtime. Click is not loud but it is audible. Sound is click free otherwise. This code is the init() of a singleton audio class.

	init() {

		do {

			print("Initializing ToneGenerator.")

			audioSession = AVAudioSession.sharedInstance()

			try audioSession.setPreferredIOBufferDuration(512 * 2 / audioSession.sampleRate)

			try audioSession.setCategory(AVAudioSession.Category.ambient) // , options: AVAudioSession.CategoryOptions())

			try audioSession.setActive(true)

		} catch let error { print("Creating audio session failed with error: \(error.localizedDescription)"); assert(false) }

		sampleRate = audioSession.sampleRate

		ioBufferDuration = audioSession.ioBufferDuration

		framesPerBuffer = Int(round(ioBufferDuration * sampleRate)) // round() is faster to type check than .rounded().

		print("audioSession.sampleRate = \(sampleRate)")

		print("audioSession.ioBufferDuration = \(audioSession.ioBufferDuration)")

		print("ioBufferDuration * sampleRate = frames per buffer = \(ioBufferDuration * sampleRate)")

		let audioComponentDescription = AudioComponentDescription(

			componentType: kAudioUnitType_Output,

			componentSubType: kAudioUnitSubType_RemoteIO,

			componentManufacturer: kAudioUnitManufacturer_Apple,

			componentFlags: 0,	 componentFlagsMask: 0 )

		do {

			try audioUnit = AUAudioUnit(componentDescription: audioComponentDescription) // On my iPhone 6s this gives error: IPCAUClient.cpp:139:ConnectToRegistrationServer: IPCAUClient: can't connect to server (-66748)

			let audioFormat = AVAudioFormat(

				commonFormat: AVAudioCommonFormat.pcmFormatInt16,

				sampleRate: Double(sampleRate),

				channels:AVAudioChannelCount( 2 ),

				interleaved: true )!

			try audioUnit.inputBusses[0].setFormat(audioFormat)

			audioUnit.outputProvider = AudioCallback

			audioUnit.isOutputEnabled = true // Default is true.

			try audioUnit.allocateRenderResources() // v2 AudioUnitInitialize()

			try audioUnit.startHardware() // v2 AudioOutputUnitStart()

		} catch let error { print("Creating audio unit failed with error: \(error.localizedDescription)"); assert(false) }
	}