Switching to 512 frames in Audiobus should resolve the issue.
GarageBand for iOS 1.4 or later: May have pops, clicks, or dropouts with Audiobus
When using GarageBand for iOS 1.4 or later with Audiobus, you might hear pops, clicks, or audio dropouts. This issue can occur when using processor-intensive applications with a buffer size of 256 frames.
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